bluuebunny/crossref_metadata_2025 huggingface dataset:
Have the Results of Randomized Clinical Trials of Pacing Altered the Practice of Cardiac Pacing?
Authors: LAMAS GERVASIO A. , WILLIAMS ADAM
Published: September 2003

Randomized clinical trials are the gold standard for the evaluation of new therapies. However, in the early years of pacing, the observational benefits were so great and the lifesaving benefits to patients so readily obvious that randomized trials were not necessary to prove benefit. As the technology has matured, advances have become more evolutionary than revolutionary, and observational analyses are unable to provide convincing evidence of small‐to‐moderate benefits. Thus, randomized trials of sufficient sample size are necessary to reliably assess the small‐to‐moderate effects of advances such as dual‐chamber pacing, rate modulation, and mode switching. It is only during the last decade, however, that the evidence base for pacing with regard to randomized trials has begun to emerge. It is unclear whether the emerging results of these clinical trials have affected the clinical practice of pacing. (J Cardiovasc Electrophysiol, Vol. 14, pp. S15‐S19, September 2003, Suppl.)
Prevention of Vascular Events in Patients with Atrial Fibrillation:
Authors: CONNOLLY STUART J.
Published: September 2003

Atrial fibrillation (AF) is a common arrhythmia that is associated with an increased risk for vascular events, particularly stroke. Two different therapies have been extensively evaluated for prevention of vascular events in AF: oral anticoagulation (such as warfarin), and aspirin. Placebo‐controlled trials of warfarin have been performed and summarized in a meta‐analysis. There is clear evidence of a benefit, with a relative risk reduction in stroke of 67% and in total vascular events of 42%. Aspirin also has been studied and is effective, but with a more modest benefit (relative risk reduction of 22%). Several studies have compared warfarin and aspirin, and showed a clear benefit in favor of warfarin for reduction of vascular events and stroke. Compared to aspirin, the risk of major hemorrhage with oral anticoagulation is increased by 70% to 100%. Current practice guidelines recommend oral anticoagulation therapy for high‐risk patients with AF, unless there is an increased risk for bleeding. Nonetheless, oral anticoagulation therapy with drugs such as warfarin is difficult for both patients and physicians because of the increased risk for bleeding and the need for ongoing monitoring of coagulation status. Many patients do not receive anticoagulation therapy despite its proven benefits. (J Cardiovasc Electrophysiol, Vol. 14, pp. S52‐S55, September 2003, Suppl.)
Use of Nonantiarrhythmic Drugs for Prevention of Sudden Cardiac Death
Authors: ALBERTE CESAR , ZIPES DOUGLAS P.
Published: September 2003

It is of interest that the drugs having the most significant impact on total and sudden death mortality are those without direct electrophysiologic actions on myocardial excitable tissue. This observation may provide insight into mechanisms responsible for ventricular tachyarrhythmias causing cardiac arrest. One way to think about ventricular fibrillation is that it is the final common pathway of an electrically unstable heart. After all, the heart can “die” in only three major ways: electromechanical dissociation, asystole and heart block, and ventricular fibrillation, with the latter most common. It is the “upstream” events provoking the electrical instability that these drugs probably act upon (i.e., ischemia, fibrosis). Although we unquestionably need to pursue investigations into the electrophysiology of these ventricular tachyarrhythmias, more studies need to investigate the drugs affecting upstream events, because these agents appear to yield the greatest dividends, at least for the present. This article reviews these drugs and how they may be effective. (J Cardiovasc Electrophysiol, Vol. 14, pp. S87‐S95, September 2003, Suppl.)
Automatic External Defibrillators for Prevention of Out‐of‐Hospital Sudden Death:
Authors: MYERBURG ROBERT J. , VELEZ MAURICIO , ROSENBERG DONALD G. , FENSTER JEFFREY , CASTELLANOS AGUSTIN
Published: September 2003

Approaches to the prevention of sudden cardiac death (SCD) include strategies designed to attack the problem from the multiple perspectives of primary prevention of the underlying diseases, prophylactic treatment of high‐risk individuals with identified diseases, and responses to cardiac arrest victims in the community. The latter strategy began with conventional fire department‐based emergency rescue systems (emergency medical services [EMS]) that originated in the early 1970s. Although such systems were innovative and impressive at the time, they are limited by less‐than‐optimal response times that translate to low survival rates. Newer strategies, designed to respond faster, include a variety of methods, including ambulance‐ and police‐based automatic external defibrillators (AEDs), deployment of AEDs in settings in which crowds accumulate and designated rescuers are available, and more general public access sites. The value of conventional EMS systems remains because of their ability to provide advanced life support as part of a dual‐response system. These approaches, in conjunction with better primary and secondary prevention strategies, offer the hope of reducing the SCD burden. (J Cardiovasc Electrophysiol, Vol. 14, pp. S108‐S116, September 2003, Suppl.)
Syncope in the Elderly:
Authors: KENNY ROSE ANNE
Published: September 2003

Syncope is more common in older persons than in any other age group. Age‐related physiologic impairments of heart rate, blood pressure, baroreflex sensitivity, and cerebral blood flow, in combination with a higher prevalence of comorbid disorders and concomitant medications, account for the increased susceptibility of older persons to syncope. A number of age‐related factors confound the assessment of syncope in older persons. Examples include (1) more than one possible attributable diagnosis; (2) polypharmacy; (3) cardiovascular and cerebrovascular comorbidity; (4) amnesia for loss of consciousness; (5) lack of witness accounts; (6) syncopal events presenting as falls; and (7) coexistent cognitive impairment and dementia. Cardiovascular risk factors are increasingly recognized as key triggers for dementia, with enormous potential for early detection and intervention. One such possible risk factor is bradyarrhythmia. Successful interventions for cardiovascular disorders that cause syncope in the elderly have wider implications than control of syncopal events and include reductions in falls and possibly prevention or modification of cognitive impairment and dementia. (J Cardiovasc Electrophysiol, Vol. 14, pp. S74‐S77, September 2003, Suppl.)
Cardiac Resynchronization Therapy is an Important Advance in the Management of Congestive Heart Failure
Authors: LECLERCQ CHRISTOPHE , DAUBERT J. CLAUDE
Published: September 2003

Cardiac resynchronization therapy (CRT) is an emerging therapy that improves symptoms and exercise tolerance in patients with advanced heart failure, left ventricular systolic dysfunction, and intraventricular conduction delay. By correcting the AV, interventricular, and intraventricular dyssynchrony induced by conduction disorders, controlled studies have shown that CRT improved functional status, decreased heart failure hospitalization rate, and might have a positive effect on left ventricular remodeling. Recent and preliminary data from the COMPANION trial suggest that CRT alone or in association with defibrillator capacity significantly reduced total mortality and hospitalization and that total mortality was significantly reduced only in the CRT plus implantable cardioverter defibrillator (ICD) group. Many questions remain unanswered, particularly the selection of responder patients. (J Cardiovasc Electrophysiol, Vol. 14, pp. S27‐S29, September 2003, Suppl.)
Analysis of Implantable Cardioverter Defibrillator Therapy in the Antiarrhythmics Versus Implantable Defibrillators (AVID) Trial
Authors: KLEIN RICHARD C. , RAITT MERRITT H. , WILKOFF BRUCE L. , BECKMAN KAREN J. , COROMILAS JAMES , WYSE D. GEORGE , FRIEDMAN PETER L. , MARTINS JAMES B. , EPSTEIN ANDREW E. , HALLSTROM ALFRED P. , LEDINGHAM ROBERT B. , BELCO KAREN M. , GREENE H. LEON ,   The Avid Investigators
Published: September 2003

Introduction: The implantable cardioverter defibrillator (ICD) is commonly used to treat patients with documented sustained ventricular tachycardia (VT) or ventricular fibrillation (VF). Arrhythmia recurrence rates in these patients are high, but which patients will receive a therapy and the forms of arrhythmia recurrence (VT or VF) are poorly understood.

Methods and Results: The therapy delivered by the ICD was examined in 449 patients randomized to ICD therapy in the Antiarrhythmics Versus Implantable Defibrillators (AVID) Trial. Events triggering ICD shocks or antitachycardia pacing (ATP) were reviewed for arrhythmia diagnosis, clinical symptoms, activity at the onset of the arrhythmia, and appropriateness and results of therapy. Both shock and ATP therapies were frequent by 2 years, with 68% of patients receiving some therapy or having an arrhythmic death. An appropriate shock was delivered in 53% of patients, and ATP was delivered in 68% of patients who had ATP activated. The first arrhythmia treated in follow‐up was diagnosed as VT (63%), VF (13%), supraventricular tachycardia (18%), unknown arrhythmia (3%), or due to ICD malfunction or inappropriate sensing (3%). Acceleration of an arrhythmia by the ICD occurred in 8% of patients who received any therapy. No physical activity consistently preceded arrhythmias, nor did any single clinical factor predict the symptoms of the arrhythmia.

Conclusion: Delivery of ICD therapy in AVID patients was common, primarily due to VT. Inappropriate ICD therapy occurred frequently. Use of ICD therapy as a surrogate endpoint for death in clinical trials should be avoided. (J Cardiovasc Electrophysiol, Vol. 14, pp. 940‐948, September 2003)
Effects of Isoproterenol and Amiodarone on the Double Potential Interval After Ablation of the Cavotricuspid Isthmus
Authors: TADA HIROSHI , ÖZAYDIN MEHMET , CHUGH AMAN , SCHARF CHRISTOPH , ORAL HAKAN , PELOSI FRANK , KNIGHT BRADLEY P. , STRICKBERGER S. ADAM , MORADY FRED
Published: September 2003

Introduction: A corridor of double potentials along the ablation line has been recognized to be an indicator of complete cavotricuspid isthmus block. Isoproterenol is used to confirm cavotricuspid isthmus block, but the effects of isoproterenol on the double potential interval (DPI), either in the absence or presence of amiodarone, are unknown.

Methods and Results: Thirty‐two patients with isthmus‐dependent atrial flutter underwent successful ablation of the cavotricuspid isthmus. The procedure was performed in the drug‐free state in 23 patients, and 2 to 7 days after discontinuation of chronic amiodarone therapy in 9 patients. Electrograms recorded along the ablation line before and during isoproterenol infusion were analyzed after isthmus block was achieved. Double potentials were recorded along the entire ablation line upon achievement of complete isthmus block in all patients. The DPI in 9 patients treated with amiodarone was longer than in the other patients (147 ± 32 msec vs 119 ± 19 msec, P < 0.001). The DPI increased as the pacing cycle length shortened in patients treated with amiodarone, but not in the other patients. At all pacing cycle lengths, isoproterenol shortened the DPI to a greater extent in the patients treated with amiodarone than in the other patients.

Conclusion: Amiodarone results in rate‐dependent prolongation of the DPI during coronary sinus pacing after ablation of the cavotricuspid isthmus. Isoproterenol shortens the DPI despite the presence of complete isthmus block, and this effect is accentuated in the presence of amiodarone. (J Cardiovasc Electrophysiol, Vol. 14, pp. 935‐939, September 2003)
Evaluation of Myocardial Performance with Conventional Single‐Site Ventricular Pacing and Biventricular Pacing in a Canine Model of Atrioventricular Block
Authors: FRIAS PATRICIO A. , CORVERA JOEL S. , SCHMARKEY LAURA , STRIEPER MARGARET , CAMPBELL ROBERT M. , VINTEN‐JOHANSEN JAKOB
Published: September 2003

Introduction: The aim of this study was to evaluate epicardial biventricular pacing as a means of maintaining synchronous ventricular activation in an acute canine model of AV block with normal ventricular anatomy and function. Chronic single‐site ventricular pacing results in dyssynchronous ventricular activation and may contribute to ventricular dysfunction. Biventricular pacing has been used successfully in adult patients with congestive heart failure.

Methods and Results: This was an acute study of open chest mongrel dogs (n = 13). ECG, left ventricular (LV), aortic, and pulmonary arterial pressures were measured. LV impedance catheters were used to assess cardiodynamics using instantaneous LV pressure‐volume relations (PVR). Following radiofrequency ablation of the AV node, a temporary pacemaker was programmed 10 beats/min above the intrinsic atrial rate, with an AV interval similar to the baseline intrinsic PR interval. The pacing protocol consisted of 5‐minute intervals with the following lead configurations: right atrium‐right ventricular apex (RA‐RVA), RA‐LV apex (LVA), and RA‐biventricular using combinations of four ventricular sites (RVA, RV outflow tract [RVOT], LVA, LV base [LVB]). RA‐RVA was used as the experimental control. LV systolic mechanics, as measured by the slope of the end‐systolic (Ees) PVR (ESPVR, mmHg/cc), was statistically greater (P < 0.05) with all modes of biventricular pacing (RA‐RVA/LVA 20.0 ± 2.9, RA‐RVA/LVB 18.4 ± 2.9, RA‐RVOT/LVA 15.1 ± 1.8, RA‐RVOT/LVB 17.6 ± 2.9) compared to single‐site ventricular pacing (RA‐RVA 12.8 ± 1.6). Concurrent with this improvement in myocardial performance was a shortening of the QRS duration (RA‐RVA 97.7 ± 2.9 vs RA‐RVA/LVA 75.7 ± 4.9, RA‐RVA/LVB 70.3 ± 4.9, RA‐RVOT/LVA 65.3 ± 4.4, and RA‐RVOT/LVB 76.7 ± 5.9, P < 0.05).

Conclusion: In this acute canine model of AV block, QRS duration shortened and LV performance improved with epicardial biventricular pacing compared to standard single‐site ventricular pacing. (J Cardiovasc Electrophysiol, Vol. 14, pp. 996‐1000, September 2003)
Prevention of Immediate Reinitiation of Atrial Tachyarrhythmias by High‐Rate Overdrive Pacing:
Authors: ISRAEL CARSTEN W. , GRÖNEFELD GERIAN , EHRLICH JOACHIM R. , LI YI‐GANG , HOHNLOSER STEFAN H.
Published: September 2003

Introduction: Immediate reinitiation of atrial tachyarrhythmia (IRAT) is an important cause of failure to maintain sinus rhythm. IRAT prevention by overdrive pacing has not been evaluated in a prospective randomized trial.

Methods and Results: Patients with a DDDRP pacemaker offering temporary atrial overdrive pacing after AT termination (Post Mode Switching Overdrive Pacing [PMOP]) were enrolled into the prospective PIRAT (Prevention of IRAT) trial if paroxysmal AT episodes occurred after implantation. PMOP was randomly activated (120 beats/min for 2 min) or inactive. After 3 months, device memory was interrogated, symptoms and quality of life assessed, and patients crossed over to the alternative treatment arm for another 3 months. Primary study endpoint was the number of AT episodes; secondary endpoints were the cumulative time in AT (AT burden), percentage of AT episodes with IRAT, symptoms, and quality of life with PMOP active versus inactive. In 37 patients (21 men; 69 ± 9 years), there was no difference in the median number of AT episodes (0.37 vs 0.34 per day), AT burden (both 1%), percentage of episodes with IRAT (30%vs 28%), symptoms, and quality of life during PMOP off versus on. With PMOP active, 29% of 439 ATs restarted during and 18% before PMOP intervention. The PMOP‐induced rate increase appeared to be associated with IRAT in 9% of AT episodes.

Conclusion: Automatic overdrive pacing after AT termination did not prevent IRAT, mainly due to insufficient overdrive suppression even at 120 beats/min and the delay between AT termination and PMOP intervention. (J Cardiovasc Electrophysiol, Vol. 14, pp. 954‐959, September 2003)
Left Heart Pacing Lead Implantation Using Subxiphoid Videopericardioscopy
Authors: ZENATI MARCO A. , BONANOMI GIANLUCA , CHIN ALBERT K. , SCHWARTZMAN DAVID
Published: September 2003

Introduction: Recent clinical data support the utility of left heart pacing. The transvenous approach for left heart pacing lead implantation is imperfect. A direct epicardial approach may have advantages, but heretofore its utility has been limited because of the requirement for thoracotomy. We sought to examine the feasibility of a method for epicardial lead implantation that did not require thoracotomy.

Methods and Results: In five large swine, percutaneous access to the epicardium was achieved with subxiphoid videopericardioscopy, using a device that marries endoscopy with a port through which pacing leads could be introduced. In each animal, standard, active fixation pacing leads were implanted onto the left atrium and ventricle. The atrial lead was implanted at the base of the appendage. The ventricular lead was implanted on the anterior, lateral, and inferior walls. Continuous direct visualization of the epicardium provided guidance for lead localization and fixation, including avoidance of complications such as trauma to epicardial coronary vessels. Capture thresholds were uniformly low. Postmortem examination demonstrated anatomically accurate, uncomplicated lead fixation.

Conclusion: Using subxiphoid videopericardioscopy, uncomplicated, anatomically accurate left heart epicardial pacing lead implantation can be achieved without thoracotomy. (J Cardiovasc Electrophysiol, Vol. 14, pp. 949‐953, September 2003)
Twenty‐Five Years of Insights into the Mechanisms of Supraventricular Arrhythmias
Authors: WELLENS HEIN J.J.
Published: September 2003

The introduction of programmed electrical stimulation of the heart and intracardiac activation mapping 35 years ago made it possible to study the site of origin or the pathway of a supraventricular tachycardia and to obtain insight into the mechanism of the tachycardia. Information from these studies has been the basis for the development of new therapies, such as arrhythmia surgery, antitachycardia pacing, and catheter ablation. Correlation of intracardiac findings with the 12‐lead ECG recorded during the tachycardia resulted in the recognition of ECG patterns characteristic of the different types of supraventricular tachycardias. Currently, gross localization of the site of origin of the arrhythmia is based on the 12‐lead ECG recorded during the arrhythmia, with fine‐tuning using intracardiac activation mapping and pacing. These developments during the past 3 decades have made accurate arrhythmia diagnosis possible and allow us to offer curative therapies to many of our patients with a supraventricular tachycardia. (J Cardiovasc Electrophysiol, Vol. 14, pp. 1020‐1025, September 2003)
Generalized transition state theory in terms of the potential of mean force
Authors: Schenter Gregory K. , Garrett Bruce C. , Truhlar Donald G.
Published: September 2003

The relationship between the free energy of activation and the potential of mean force is derived for reaction coordinates that are arbitrary functions of all the coordinates defining a system. The general result is illustrated for rectilinear reaction coordinates, bond distance reaction coordinates, atom transfer reaction coordinates, synchronous double atom transfer reaction coordinates, and energy gap reaction coordinates.
The accuracy of density functionals for electric field gradients. Test calculations for ScX, CuX and GaX (X=F, Cl, Br, I, H and Li)
Authors: Bast Radovan , Schwerdtfeger Peter
Published: September 2003

In a previous paper [J. Chem. Phys. 111, 3357 (1999)] we showed that the electric field gradient at the copper nucleus in CuCl is incorrectly described by most of the density functionals currently in use, including gradient corrected and hybrid versions of DFT. Here we analyze whether this error is systematic or not by comparing DFT electric field gradients for a number of diatomic compounds MX. The molecules chosen include representatives from early transition metal compounds, ScX, from late transition metal compounds, CuX, and from main group compounds, GaX, where X=F, Cl, Br, I, H and Li. From experimental nuclear quadrupole coupling data and electric field gradient calculations for each of the three sets (ScX, CuX and GaX) the nuclear quadrupole moment at the metal can be deduced at a specific DFT level. It is demonstrated that density functionals work well for main group compounds (GaX), but contain large systematic errors for transition metals such as copper. This leads to unreasonable copper nuclear quadrupole moments obtained for the CuX set. The error mainly originates from the incorrect description of the Cu(3d) core being polarized by electronegative ligands such as fluorine. For copper this can be remedied in an ad-hoc way by adjusting the Hartree–Fock contribution in the exchange part of the hybrid functional (modified B3LYP). Using this modified B3LYP functional we obtain reasonable results for the copper electric field gradient in CuF3, a case where the Cu(3d) core is strongly polarized by the fluorine ligands.
Solvation of the hydronium ion at the water liquid/vapor interface
Authors: Dang Liem X.
Published: September 2003

In this study, we used constrained molecular dynamics techniques to investigate the transport of a hydronium ion across the water liquid/vapor interface. The computed transfer free energy was nearly unchanged as the hydronium ion approached the Gibbs dividing surface. The ion crossed the interface with no substantial minimum free energy, and transport of the hydronium ion involved a change in the solvent composition of the solvation shells around the ion.
From ab initio quantum chemistry to molecular dynamics: The delicate case of hydrogen bonding in ammonia
Authors: Boese A. Daniel , Chandra Amalendu , Martin Jan M. L. , Marx Dominik
Published: September 2003

The ammonia dimer (NH3)2 has been investigated using high-level ab initio quantum chemistry methods and density functional theory. The structure and energetics of important isomers are obtained to unprecedented accuracy without resorting to experiment. The global minimum of eclipsed Cs symmetry is characterized by a significantly bent hydrogen bond which deviates from linearity by as much as ≈20°. In addition, the so-called cyclic C2h structure, resulting from further bending which leads to two equivalent “hydrogen bonding contacts,” is extremely close in energy on an overall flat potential energy surface. It is demonstrated that none of the currently available [generalized gradient approximation (GGA), meta-GGA, and hybrid] density functionals satisfactorily describe the structure and relative energies of this nonlinear hydrogen bond. We present a novel density functional, HCTH/407+, which is designed to describe this sort of hydrogen bond quantitatively on the level of the dimer, contrary to, e.g., the widely used BLYP functional. This improved generalized gradient approximation functional is employed in Car–Parrinello ab initio molecular dynamics simulations of liquid ammonia to judge its performance in describing the associated liquid. Both the HCTH407+ and BLYP functionals describe the properties of the liquid well as judged by analysis of radial distribution functions, hydrogen bonding structure and dynamics, translational diffusion, and orientational relaxation processes. It is demonstrated that the solvation shell of the ammonia molecule in the liquid phase is dominated by steric packing effects and not so much by directional hydrogen bonding interactions. In addition, the propensity of ammonia molecules to form bifurcated and multifurcated hydrogen bonds in the liquid phase is found to be negligibly small.
Ab initio investigation of the ground state properties of PO, PO+, and PO−
Authors: Metropoulos Aristophanes , Papakondylis Aristotle , Mavridis Aristides
Published: September 2003

We have computed accurate potential energy curves of the ground states of the PO(X 2Π), PO+(X 1Σ+) and PO−(X 3Σ−) species by multireference configuration interaction and coupled-cluster methods and have obtained accurate spectroscopic constants for each species. We have also determined the effect of core on the properties above and have obtained the PO complete basis set limit by the multireference method for the equilibrium energy, bond distance, dissociation energy, harmonic frequency, and dipole moment.
A critical analysis of the ground and excited electronic states of transition metal nitrides using the relativistic effective Hamiltonian method
Authors: Chaudhuri Rajat K. , Freed Karl F.
Published: September 2003

Multireference many-body perturbative schemes (IVO–CASCI and Hv3rd), which are applicable to the direct calculation of excitation energies, ionization potentials, and spectroscopic properties, are presented and applied to compute the transition energies, ionization potentials, and spectroscopic constants of TiN and VN. Highly satisfactory results are obtained for the excitation energies, triple bond dissociation energy, dipole moments, oscillator strengths, and vibrational frequencies. The ground and excited properties of interest are also computed using Hartree–Fock and two-component Dirac–Hartree–Fock molecular orbitals to assess the importance of relativistic effects. We also report the ionization potentials of TiN+ and VN+ which are by-products of this method with “no extra” computational cost and which have not been studied previously.
Non-nuclear maxima of the electron density on alkaline metals
Authors: Luaña Vı́ctor , Mori-Sánchez Paula , Costales Aurora , Blanco M. A. , Pendás A. Martı́n
Published: September 2003

The topological properties of the electron density of bcc alkaline metals (Li–Cs) is examined by means of Hartree–Fock and density functional calculations. Our best results indicate that lithium is the only alkaline metal showing non-nuclear maxima (NNM) at the room pressure and temperature experimental geometry. Sodium and potassium, but not rubidium and cesium, would also present NNM under an appropriate compression, even though the NNM in potassium would be residual at best and contain a negligible amount of electrons. Despite these differences, all five alkaline metals share a common tendency towards topological change that makes their behavior clearly distinct from what is typical in ionic, covalent and molecular crystals. When examined in a wide range of interatomic distances, the electron density of every metal follows a well defined topological sequence, with strong similarities across the five metals.
Modeling shear-induced phase transitions of binary polymer mixtures
Authors: Jupp Laurence , Kawakatsu Toshihiro , Yuan Xue-Feng
Published: September 2003

The shear-induced phase transitions of binary polymer mixtures have been studied. We solved the two-fluid model numerically in a high Weissenberg number and infinite Peclet number flow regime without linearization of the diffusive equation or the adiabatic approximation. Our simulations have captured key features of flow-induced phase transitions, such as the shift in the phase boundary from the equilibrium phase diagram under flow conditions. Both shear-induced mixing and demixing phenomena have been reproduced. The dynamic phase behavior of polymer mixtures is governed by parameters including φA and χ for thermodynamic forces, τ′ and G′ for the degree of dynamical asymmetry in polymer mixtures, and the imposed nominal shear rate. Dynamic phase diagrams are obtained for steady state flow. We have identified correlations between the microstructure evolution and rheology, and a mechanism to reach the steady state. The rheological signature of demixing fluids under shear is that the dependence of the steady state shear stress on shear rate changes from nonlinear to linear within that range of shear rates in which the demixing occurs.
Solvation of excess electrons in supercritical ammonia
Authors: Rodriguez Javier , Skaf Munir S. , Laria Daniel
Published: September 2003

Molecular dynamics simulations have been performed to study equilibrium and dynamical aspects of solvation of excess electrons in supercritical ammonia along the T=450 K isotherm. The interval of supercritical densities investigated spans from typically dense liquid down to dilute vapor ambients. Equilibrium aspects of solvation were analyzed using combined path integral-molecular dynamics techniques. The transition from localized to quasifree states, described in terms of the isomorphic electron-polymer spatial extent, was observed at approximately one fourth of the triple point density, a value somewhat higher than that recently reported for supercritical water [D. Laria and M. Skaf, J. Phys. Chem. A 106, 8066 (2002)]. The density of electronic eigenstates shows typically one s- and three p-like bound states that gradually lose their symmetry characteristics as the density lowers. The computed ground state absorption spectrum exhibits redshifts in the absorption bands as the density decreases; these shifts are much larger than those reported by pulse radiolysis experiments. By performing adiabatic dynamics, we also investigate mechanisms for solvent relaxation at high and intermediate supercritical densities following a vertical excitation of the electron.
Molecular dynamics investigation on liquid–liquid phase change in carbon with empirical bond-order potentials
Authors: Kum Oyeon , Ree Francis H. , Stuart Steven J. , Wu Christine J.
Published: September 2003

A liquid–liquid phase transition in carbon is investigated with two recent bond-order potentials. In contrary to a previous bond-order model, they show no phase change in liquid carbon, which agrees with simulations based on the nonempirical density-functional theory (DFT). Ab initio and DFT studies carried out in this work show that the observed discrepancy lies not in any inherent shortcoming in using empirical models for the bonding process, but rather in the quality of individual expressions used to represent a conjugated local environment in liquid carbon. The present work shows that the current bond-order models and a slightly modified potential proposed in this work agree with recent quantum mechanical simulations and will provide a viable tool for a large-scale study of carbon over a wide range of pressures and temperatures.
A new method for solving the quantum hydrodynamic equations of motion
Authors: Kendrick Brian K.
Published: September 2003

The quantum hydrodynamic equations associated with the de Broglie–Bohm formulation of quantum mechanics are solved using a meshless method based on a moving least squares approach. An arbitrary Lagrangian–Eulerian frame of reference is used which significantly improves the accuracy and stability of the method when compared to an approach based on a purely Lagrangian frame of reference. A regridding algorithm is implemented which adds and deletes points when necessary in order to maintain accurate and stable calculations. It is shown that unitarity in the time evolution of the quantum wave packet is significantly improved by propagating using averaged fields. As nodes in the reflected wave packet start to form, the quantum potential and force become very large and numerical instabilities occur. By introducing artificial viscosity into the equations of motion, these instabilities can be avoided and the stable propagation of the wave packet for very long times becomes possible. Results are presented for the scattering of a wave packet from a repulsive Eckart barrier.
Hyper-Rayleigh scattering and frequency dependence of the first molecular hyperpolarizability of a strong charge-transfer chromophore
Authors: Wang C. H. , Lin Y. C. , Tai Oliver Y. , Jen Alex K.-Y.
Published: September 2003

The frequency dependence of the first molecular hyperpolarizability β of a thiophene based charge-transfer chromophore has been measured by using a number of excitation wavelengths emitted from an optical parametric amplifier pumped by a 1 KHz 130 fs Ti/sapphire laser. The excitation wavelength covers the resonance and nonresonance region of the charge transfer chromophore. β shows a maximum on the red side of the charge transfer electronic absorption band. A comparison of the experimental result with a recent proposed calculation involving the Kramers–Kronig transformation of the charge-transfer absorption spectrum is made. It is found that while the Kramers–Kronig transformation analysis predicts a correct position of the β maximum, it does not yield an agreement in the resonance region of the frequency dependent first molecular hyperpolarizability.
Quasiclassical and semiclassical wave-packet dynamics in periodic potentials
Authors: Balzer Birgit , Dilthey Stefan , Stock Gerhard , Thoss Michael
Published: September 2003

The capability of quasiclassical and semiclassical methods to describe quantum dynamics in a periodic potential is investigated. Due to the periodicity of the potential, such systems may exhibit prominent quantum interference effects and, therefore, provide a particular challenge to methods based on classical approximations. Adopting a simple model for an isomerization reaction, we study the dynamics for different initial preparations as well as different dynamical observables. The quasiclassical calculations are based on the classical Wigner method and the semiclassical approach utilizes the Herman–Kluk (coherent state) initial-value representation, generalized to properly take into account the boundary conditions of the wave functions in a periodic potential. The results of the study show that the quasiclassical method can only describe the quantum dynamics in situations where the system is confined to the potential well and for highly averaged observables but fails otherwise. The semiclassical method, on the other hand, provides an excellent description of the quantum dynamics as long as the initial state is energetically separated from the torsional barrier. The reasons why the quasiclassical and semiclassical methods perform well in some situations but fail for others are discussed in some detail. Furthermore, the relation between the performance of the quasiclassical and semiclassical methods and the eigenvalue structure of the participating eigenstates is analyzed.
Potential energy surfaces for the uranium hydriding reaction
Authors: Balasubramanian K. , Siekhaus Wigbert J. , McLean William
Published: September 2003

We have computed the potential energy surfaces for the low-lying electronic states of uranium hydrides, UHn (n=1–3), which are important in the uranium hydriding reactions. We have employed a number of computational methods including the complete active space multiconfiguration self-consistent field followed by multireference relativistic configuration interaction computations with spin–orbit coupling that included up to 6 million configurations. We find that the activation barrier to insert uranium into H2 is reduced substantially by spin–orbit coupling, and the product species UH2 in its A1 spin–orbit ground state is substantially stable over U(5L)+H2 dissociated products. We have found two electronic states for UH to be quite close to each other, and depending on the level of theory the relative ordering of the Λ6 and I4 states changes, I4 state being the lowest at the highest second-order configuration interaction level. The UH2 species also exhibits a similar feature in that the triplet state is favored at the single-reference second-order Møller–Plesset and coupled cluster levels, while the quintet state is favored at the multireference and density functional theory levels. The UH3 species is extremely floppy, exhibiting an inversion potential surface that has a barrier smaller than its zero-point energy. It is shown that the UH3 species is considerably more ionic than UH2 or UH, and UH3 is responsible for catalyzing the U-hydriding reaction as the highly positive U site in UH3 reacts with H2 spontaneously without an activation barrier. The results of our computations are compared with previous experimental results. The spin–orbit coupling is shown to be more important for energy activation than near the minima.
Møller–Plesset perturbation theory for vibrational wave functions
Authors: Christiansen Ove
Published: September 2003

The vibrational self-consistent field wave function is considered as the zeroth order state in a vibrational Møller–Plesset (VMP) perturbation theory. A method for calculating the contributions to arbitrary order is described and implemented. The theoretical background for understanding and analyzing the behavior of convergent and divergent VMP expansions is discussed briefly. Examples of convergent and divergent vibrational Møller–Plesset perturbation series are given and analyzed for two-mode model systems and for a formaldehyde quartic force field. It is found that direct use of high order VMP is problematic for calculation of anharmonic vibrational energies.
Effects of initial state preparation on the distance dependence of electron transfer through molecular bridges and wires
Authors: Skourtis Spiros , Nitzan Abraham
Published: September 2003

The dependence of electron transfer rates and yields in bridged molecular systems on the bridge length, and the dependence of the zero-bias conduction of molecular wires on wire length are discussed. Both phenomena are controlled by tunneling across the molecular bridge and are consequently expected to show exponential decrease with bridge length that is indeed often observed. Deviations from this exponential dependence for long bridges, in particular a crossover to a very weak dependence on bridge length, were recently observed experimentally and discussed theoretically in terms of thermal relaxation and dephasing on the bridge. Here we discuss two other factors that potentially affect the bridge length dependence of these phenomena. First, in experiments initiated by an initial preparation of a nonstationary “donor” state the initial energy is not well defined. A small contribution from initially populated eigenstates that are of mostly bridge-level character may dominate transmission for long bridges, resulting in weak or no bridge-length dependence. Second, in steady state experiments the distribution of initial states (for example, the Fermi distribution at the electrodes in conduction experiments) will cause deviations from exponential dependence on wire length because even a small population in higher energy states will dominate the transmission through long wires. For the first scenario we show that the crossover behavior observed for electron transfer in DNA between G and GGG species separated by AT chains can be largely reproduced just by initial state effects.
Quantitative study of the encapsulation of glucose oxidase into multilamellar vesicles and its effect on enzyme activity
Authors: Olea David , Faure Chrystel
Published: September 2003

The encapsulation of glucose oxidase (GOx) into onion-type multilamellar vesicles is studied and compared to that of GOx into liposomes. The enzyme was shown not to be affected by encapsulation as evidenced by the complete recovery of its activity after being freed. An ∼15% increase of GOx activity was conferred by confinement in onions in the 30–50 °C temperature range. Entrapment of GOx in onions was proved to be effective since a maximum of 10% leak was measured after 45 days of encapsulation. The encapsulation yield, which reaches 80%, and the number of encapsulated enzyme molecules per onion (1000 GOx molecules) were found to be much higher than for liposomes. The effect of onion composition on the encapsulation yield was determined and predicted by a thermodynamic model applied to the lipids–GOx–phosphate buffer system.
Model for the translation–rotation coupling of molecular motion in water
Authors: Faraone A. , Liu L. , Chen S.-H.
Published: September 2003

Using molecular dynamic simulations of an extended-simple-point-charge model of water, we investigated the effect of translation–rotation coupling on the motion of the hydrogen atom in a water molecule in low temperature water. We introduce a translation–rotation probability distribution PCM,Ω(r,θR,t), where r(t) is the scalar displacement of the center of mass from origin at time t starting from the origin at t=0; and θR(t)=arccos[b(t)⋅b(0)/b2] is the angle of rotation of the vector, b, connecting the center of mass of the water molecule to the hydrogen atom position. We developed a procedure to determine PCM,Ω(r,θR,t) starting from the van Hove self-correlation function of the center of mass, GsCM(r,t), taking into account the translation–rotation coupling effect. In the long time region, we use a translation–rotation coupling parameter, Q*, to determine the angular dependence of PCM,Ω(r,θR,t) from GsCM(r,t). Thus, the dynamics of the hydrogen atoms in a water molecule is fully determined by the van Hove self-correlation function of the center of mass and by the value of the translation–rotation coupling parameter. In particular, we show that the self intermediate scattering function of the hydrogen atom can be calculated with a better accuracy than using the traditional decoupling approximation. The model can be used to analyze incoherent quasielastic neutron scattering spectra of water in the future.
A rate constant expression for charge transfer through fluctuating bridges
Authors: Troisi Alessandro , Nitzan Abraham , Ratner Mark A.
Published: September 2003

A rate constant expression for charge transfer reactions mediated by flexible bridges is presented as a series of terms of decreasing importance. The leading term corresponds to the static limit obtained from the Condon approximation. Corrections due to finite time fluctuations are evaluated explicitly, assuming a Gaussian shape of the coupling autocorrelation function and the Marcus model with a one-dimensional harmonic thermal bath. The use of this model for the interpretation of experimental data and the expected magnitudes of the fluctuation effects are discussed.
Temperature-dependent Raman spectra of triaminotrinitrobenzene: Anharmonic mode couplings in an energetic material
Authors: McGrane S. D. , Shreve A. P.
Published: September 2003

The temperature-dependent Raman spectrum of triaminotrinitrobenzene (TATB) is examined to determine phonon–molecular vibrational coupling information. Spectra are reported from 4–295 K over a spectral range 50–3350 cm−1. The 4 K TATB spectrum allows far greater resolution than at room temperature, with sharp phonon peaks prominent. The temperature dependence in TATB is fit very well by standard perturbation theoretical expressions for cubic (peak broadening) and quartic (peak frequency shift) anharmonicities, and averaged mode coupling information can thus be obtained. Confident assignments of specific mode coupling constants are not possible due to the large number of potentially interacting vibrations. However, these data on the entire spectrum across the broad range of temperatures provide experimental constraints useful for testing models of vibrational energy transfer in TATB. Possible relations of this study to shock initiation are briefly discussed.
An effective model for the X 2A1–A 2B2 conical intersection in NO2
Authors: Joyeux M. , Jost R. , Lombardi M.
Published: September 2003

We propose an efficient method for calculating the eigenstates and adjusting the parameters of an effective Hamiltonian, which reproduces the experimentally observed energy levels of NO2 up to 11 800 cm−1 above the quantum mechanical ground state, that is a few thousands of cm−1 above the X 2A1–A 2B2 conical intersection, with a rms error less than 4 cm−1. This method principally relies on the determination, through first-order perturbation theory, of an optimal basis for each surface, which takes into account the nonresonant energy shifts experienced by the states of this surface. As a result, the size of the matrix, which one has to build and diagonalize to converge the spectrum up to 11 800 cm−1, is of the order of 500–1000 instead of several tens of thousands. Thank to this Hamiltonian, the analysis of the experimental spectrum up to 11 800 cm−1 could be completed. A detailed description of all states located above 9500 cm−1 is proposed, those lying below 9500 cm−1 being already known and tabulated.
A statistical, ab initio, quantum mechanical study of the photolysis and final state distributions of singlet ketene
Authors: Cole John P. , Balint-Kurti Gabriel G.
Published: September 2003

A new quantum mechanical, statistical, total angular momentum conserving theory designed to describe relative kinetic energy and fragment quantum state distributions in unimolecular dissociation processes is described. The theory is called the statistical adiabatic product distribution method and is based on a variational Rice–Ramsperger–Kassel–Marcus (RRKM) treatment of the break-up process. It requires the definition of a break-up pathway or intrinsic reaction coordinate and the normal mode vibrational frequencies in the coordinate space orthogonal to this coordinate. In the present application to the break up of highly excited singlet ketene, the reaction coordinate and vibrational frequencies are evaluated using ab initio molecular electronic structure codes. The variational aspect of the theory involves locating, independently for every total angular momentum and total energy, the reaction coordinate value which leads to the lowest sum-of-states. In order to make predictions of the product quantum state and relative kinetic energy distributions the variational RRKM treatment is augmented by a J conserving quantum phase space treatment of the dissociation process. This treatment also takes into account the variation of the electronic energy along the reaction coordinate during the final stage of the break-up process. The conserved modes of the molecule are treated adiabatically during the break-up process as the fragment separation increases beyond the position of the critical geometry. The quantum phase space theory treatment enables us to identify the energy associated with rotation and translation at the critical geometry. The rotational motion of the fragments is also treated adiabatically during the break-up process, while the relative translational energy is used to surmount the potential and centrifugal barrier which may still have to be overcome to permit the fragments to separate. The phase space theory used takes proper account of the limitations placed on the dynamics by the necessity for the system to surmount these barriers. Extensive investigations are carried out as to whether the transition state or critical geometry arising in the variational RRKM treatment is best computed by treating the system as a bound molecule or as two separated fragments. For the dissociation of ketene at the high levels of excitation treated in the present work, we conclude that the variational calculation of the critical geometry is best performed by treating the system as a bound molecule and using the ab initio computed normal mode vibrational frequencies in the coordinate space perpendicular to the reaction path.