# Collision-Induced Infrared Absorption by Molecular Hydrogen Pairs at Thousands of Kelvin

Authors: Xiaoping Li; Katharine L. C. Hunt; Fei Wang; Martin Abel; Lothar Frommhold
Journal: International Journal of Spectroscopy
 (2010)

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## Abstract

Collision-induced absorption by hydrogen and helium in the stellar atmospheres of cool white dwarfs causes the emission spectra to differ significantly from the expected blackbody spectra of the cores. For detailed modeling of radiative processes at temperatures up to 7000 K, the existingH2–H2 induced dipole and potential energy surfaces of high quality must be supplemented by calculations with the H2 bonds stretched or compressed far from the equilibrium length. In this work, we describe new dipole and energy surfaces, based on more than 20  000 ab initio calculations for H2–H2. Our results agree well with previous ab initio  work (where those data exist); the calculated rototranslational absorption spectrum at 297.5 K matches experiment similarly well. We further report the calculated absorption spectra of H2–H2 for frequencies from the far infrared to 20 2009;000 cm−1, at temperatures of 600 K, 1000 K, and 2000 K, for which there are no experimental data.


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## 1. Introduction

It is well known that dense gases of infrared inactive molecules such asH2 absorb infrared radiation. Absorption continua range from the microwave and far infrared regions of the spectrum to the near infrared and possibly into the visible. Collisionally interacting pairs of hydrogen molecules possess transient electric dipole moments, which are responsible for the observed absorption continua [1, 2]. Planetary scientists understood early on the significance of collision-induced absorption (CIA) for the modeling of the atmospheres of the outer planets [3, 4]. More recently, it was shown that the emission spectrum of cool white dwarf stars differs significantly from the expected blackbody spectrum of their cores: CIA in the dense helium and hydrogen atmospheres suppresses (filters) the infrared emissions strongly [5–10]. Detailed modelling of the atmospheres of cool stars with proper accounting for the collision-induced opacities is desirable, but it has been hampered heretofore by the highly incomplete or nonexisting theoretical and experimental data on such opacities at temperatures of many thousands of kelvin.Quantum chemical calculations of the induced dipole surfaces ofH2–H2, H2–He and other complexes have been very successful [11–14]. Based on such data, molecular scattering calculations accounting for the interactions of the molecular complexes with photons have been under-taken which accurately reproduced the existing laboratory measurements at low temperatures (T ≤ 300 K or so) [2]. At higher temperatures, virtually no suitable laboratory measurements of such opacities exist, but reliable data are needed. We therefore decided to extend such quantum chemical calculations of the induced dipole (ID) and potential energy surfaces (PES) of H2–H2 complexes to highly rotovibrationally excited molecules, as encountered at high temperatures (up to 7 000 K) and photon energies up to ~2.5 eV.


## 2.Ab InitioCalculations of the Induced Dipole and Potential Energy Surfaces

At the temperatures characteristic of cool white-dwarf atmospheres, the CIA spectra depend on transition dipole matrix elements with vibrational quantum numbers up tov≈7. To evaluate these matrix elements, we have determined the induced dipoles and interaction energies of pairs of hydrogen molecules with bond lengths ranging from 0.942 a.u. to 2.801 a.u. (1 a.u. =a0=5.29177249·10-11 m). For comparison, the vibrationally averaged internuclear separation in H2 is 1.449 a.u., in the ground vibrational state. We have used MOLPRO 2000 [15] to calculate the PES for H2–H2 and to calculate the pair ID by finite-field methods, at coupled-cluster single and double excitation level, with triple excitations treated perturbatively [CCSD(T)]. In this work, we have employed MOLPRO’s aug-cc-pV5Z(spdf) basis, consisting of (9s 5p 4d 3f) primitive Gaussians contracted to [6s 5p 4d 3f]; this gives 124 contracted basis functions for each of the H2 molecules. The basis gives accurate energies and properties [16]; yet it is sufficiently compact to permit calculations on H2 pairs with 28 different combinations of H2 bond lengths, at 7 different intermolecular separations, in 17 different relative orientations (the orientations listed in Table 1), and at a minimum of 6 different applied field strengths for each geometrical configuration.Table 1
Cartesian componentsμX, μY, and μZ of the H2–H2 dipole in a.u. (multiplied by 106) with bond lengths r1=r2=1.449 a.u.
R (a.u.)4.05.06.07.08.09.010.0(θ1, θ2, φ12)μX(π/12, π/6, π/3)4815010363392516(π/12, π/4, π/6)−3675−2393−1393−804−480−299−195(π/12, π/3, π/6)−2790−1791−1044−607−366−230−150(π/12, 5π/12, π/6)−144−28−17−16−15−11−8(π/6, π/4, π/3)14171294806471280175115(π/6, π/3, π/4)3995623652101217449(π/6, 5π/12, π/3)206519221196695411255167(π/4, π/3, π/6)110987952830217710972(π/4, 5π/12, π/6)348124321424815481299195(π/3, 5π/12, π/6)255518041062611363226148(7π/12, π/12, π/6)−7979−5226−3027−1740−1037−648−424(7π/12, π/6, π/4)−9089−5973−3462−1988−1184−740−484(7π/12, π/4, π/6)−11040−7181−4151−2381−1417−885−580(7π/12, π/3, π/6)−9759−6345−3669−2107−1255−785−515(π/2, π/12, π/6)−3628−2337−1341−765−452−282−184(π/2, π/6, π/3)−3575−2303−1322−754−447−278−182(π/2, π/4, π/6)−7071−4535−2606−1489−884−551−361μY(π/12, π/6, π/3)−6236−4288−2519−1453−864−539−352(π/12, π/4, π/6)−3691−2635−1566−908−542−338−221(π/12, π/3, π/6)−2801−2088−1257−733−440−275−180(π/12, 5π/12, π/6)−1060−952−599−355−214−135−88(π/6, π/4, π/3)−5443−4082−2455−1429−854−533−349(π/6, π/3, π/4)−2847−2414−1496−880−529−332−217(π/6, 5π/12, π/3)−1427−1430−916−545−330−207−136(π/4, π/3, π/6)−1008−1209−793−473−286−179−117(π/4, 5π/12, π/6)419−239−224−145−90−57−37(π/3, 5π/12, π/6)404−211−199−127−78−49−32(7π/12, π/12, π/6)−997−877−543−317−189−118−77(7π/12, π/6, π/4)−2588−2187−1342−780−464−290−189(7π/12, π/4, π/6)−2416−1890−1142−662−394−246−161(7π/12, π/3, π/6)−2415−1757−1045−604−360−225−147(π/2, π/12, π/6)−835−798−501−294−175−109−71(π/2, π/6, π/3)−2521−2367−1481−865−515−321−210(π/2, π/4, π/6)−1734−1570−976−569−339−211−138μZ(π/12, π/6, π/3)−15702−5371−2141−1026−568−345−223(π/12, π/4, π/6)−35330−12145−4900−2374−1322−808−525(π/12, π/3, π/6)−53105−18342−7486−3664−2053−1258−820(π/12, 5π/12, π/6)−65061−22550−9278−4573−2574−1580−1032(π/6, π/4, π/3)−19683−6793−2764−1349−755−463−301(π/6, π/3, π/4)−37478−13007−5355−2641−1486−914−597(π/6, 5π/12, π/3)−49514−17248−7156−3553−2008−1237−810(π/4, π/3, π/6)−17837−6231−2596−1293−731−451−296(π/4, 5π/12, π/6)−29903−10485−4400−2205−1253−774−509(π/3, 5π/12, π/6)−12057−4257−1805−913−522−323−213(7π/12, π/12, π/6)653012260092864573257315801032(7π/12, π/6, π/4)49757172947161355320081237810(7π/12, π/4, π/6)3012510528440422061253775510(7π/12, π/3, π/6)1213342721807914522323214(π/2, π/12, π/6)693102404299164898276116971109(π/2, π/6, π/3)53764187467796387921961354887(π/2, π/4, π/6)342121199850422532144189158In the calculations, the centers of mass of the twoH2 molecules are separated along theZaxis by distances R ranging from 4.0 to 10.0 a.u. The vector R joins molecule 2 to molecule 1. The molecular orientations are characterized by the angles (θ1, θ2, φ12), where θ1 is the angle between the Z axis and the symmetry axis of molecule 1, θ2 is the angle between the Z axis and the symmetry axis of molecule 2, and φ12 is the dihedral angle between two planes, one defined by the Z axis and the symmetry axis of molecule 1 and the other defined by the Z axis and the symmetry axis of molecule 2.Calculations were performed first for two molecules with bond lengths ofr1=r2=1.449 a.u., the ground-state, vibrationally averaged internuclear separation. The interaction energies were evaluated in the absence of an applied field; then the pair dipoles were obtained from finite-field calculations, grouped into three sets of 40. Within each of the sets, the fields were confined to the XY, XZ, or YZ planes, and the two components of the applied field were selected randomly, in the range from 0.001 a.u. to 0.01 a.u., for a total of 120 calculations. For each fixed set of the bond lengths, orientation angles, and intermolecular separation, the total energies were fit (by least squares) to a quartic polynomial in the applied field F:(1)E=E0-μαFα-(12)ααβFαFβ-16βαβγFαFβFγ-124γαβγδFαFβFγFδ-⋯,
where the Einstein convention of summation over repeated Greek subscripts is followed. The coefficients of the linear terms were selected from each fit, to obtain the Cartesian components of the induced dipole moments μX, μY, and μZ. In Table 1, our results for the components of the pair dipole are given for pairs with r1=r2=1.449 a.u.In earlier work on the polarizabilitiesα for H2–H2 [16], we conducted several tests of this fitting procedure: we compared results from quartic fits with 120 different field strengths, quartic fits with 200 different field strengths, and quintic and sixth-order fits with 200 field strengths (at one set of orientation angles and an intermolecular distance of 2.5 a.u., where the differences between the calculations were expected to be magnified); we found excellent agreement among the results from all of the fits. We also compared the results from the random-field calculations with the values obtained analytically, based on calculations with 6 or 8 selected values of the field strengths, for fixed orientation angles and the full range of intermolecular separations. The field values were grouped into the sets {f, 21/2f, 31/2f, -f, -21/2f, -31/2f}, {f, 21/2f, 51/2f, -f, -21/2f, -51/2f}, and {f, 21/2f, 31/2f, 51/2f, -f, -21/2f, -31/2f, -51/2f}, with f= 0.001, 0.002, 0.003, and 0.004 a.u. At the shortest intermolecular distance (R=2.5 a.u.), the results for f = 0.001 a.u. – 0.003 a.u. were affected by numerical imprecision in the hyperpolarization contributions; at larger R, they agreed well with the random-field results. Agreement between the random-field results and the results obtained with f = 0.004 a.u. was excellent for all R values. On this basis, we have used random-field fits in the work with r1=r2=1.449 a.u., but we have used analytic fits with 6 different field values for the computations with r1 or r2≠1.449 a.u. In [16], we also compared the results obtained via analytic differentiation at the self-consistent field (SCF) level using Gaussian 98 versus the results from our SCF calculations, for the full range of intermolecular separations and three different relative orientations, again with excellent agreement. Basis set superposition error (BSSE) has been shown to be negligible [16], as tested by function counterpoise (“ghost-orbital”) methods. BSSE occurs when the pair basis provides a better representation of H2–H2 than the single-molecule basis provides for an isolated H2 molecule. In these calculations, BSSE has been suppressed by the large size of the single-molecule basis.The interaction mechanisms that determine the induced dipole include classical multipole polarization, van der Waals dispersion, and short-range exchange, overlap, and orbital distortion. At long range, the leading term in the collision-induced dipole comes from quadrupolar induction, which varies asR-4 in the separation R between the molecular centers [2]. The next long-range polarization term is of order R-6; it results both from hexadecapolar induction and from the effects of the nonuniformity of the local field gradient (due to the quadrupole moment of the collision partner). The magnitude of the latter term depends on the dipole-octopole polarizability tensor E. At order R-7, back-induction [17, 18] and dispersion [17–21] affect the pair dipole. Back-induction is a static reaction field effect: the field from the permanent quadrupole of molecule 1 polarizes molecule 2, which sets up a reaction field that polarizes molecule 1 (and similarly, with molecules 1 and 2 interchanged). The van der Waals dispersion dipole results from dynamic reaction-field effects, combined with the effects of an applied, static field [21], via two physical mechanisms. (1) Spontaneous, quantum mechanical fluctuations in the charge density of molecule 1 produce a fluctuating field that acts on molecule 2; then molecule 2 is hyperpolarized by the concerted action of the field from 1 and the applied field F. This sets up a field-dependent dynamic reaction field at molecule 1, giving a term in the van der Waals energy that is linear in the applied field F. (2) The correlations of the fluctuations in the charge density of molecule 1 are altered by the static field F acting on 1; molecule 2 responds linearly to field-induced changes in the fluctuations of the charge density of 1, again giving a term in the van der Waals energy that is linear in the applied field F. The precise functional forms of the short-range exchange, overlap, and orbital-distortion effects on the dipole are not known; however, these contributions are expected to drop off (roughly) exponentially with increasing R [2].The dipole moment of the pair can be cast into a symmetry-adapted form, as a series in the spherical harmonics of the orientation angles of molecules 1 and 2 and the orientation angles of the intermolecular vector:(2)μ1M(R,r1,r2)=(4π)3/23-1/2∑Aλ1λ2ΛL(R,r1,r2)×Yλ1m1(Ω1)Yλ2m2(Ω2)YLM-m(ΩR)×〈λ1λ2m1m2∣Λm〉〈ΛLm(M-m)∣1M〉,
where the sum runs over all values of λ1, λ2, m1, m2, Λ and m; M=1, 0, or -1, corresponding to the dipole components,(3)μ11=-(12)1/2(μX+iμY),μ10=μZ,μ1-1=(12)1/2(μX-iμY).
In (2), Ω1 and Ω2 denote the orientation angles of molecules 1 and 2, that is, the orientation angles of the z axes of the molecule-fixed frames, ΩR is the orientation angle of the vector R (note that R runs from molecule 2 to molecule 1, in this work), and the quantities 〈λ1λ2m1m2∣Λm〉 and 〈ΛLM(M-m)∣1M〉 are Clebsch-Gordan coefficients. Equation (2) follows immediately from the fact that the collision-induced dipole of H2–H2 is a first-rank spherical tensor, which is obtained by coupling functions of r1,r2, and R. Therefore λ1, λ2, Λ, L, and the magnitudes of r1, r2, and R completely determine the dipole expansion coefficients Aλ1λ2ΛL(R,r1,r2).The dipole coefficients arising from various long-range polarization mechanisms are categorized in Table2, through order R-7. In this table, Θ denotes the molecular quadrupole moment; α̅ is the trace of the single-molecule polarizability; α∥-α⊥ is the polarizability anisotropy, which is equal to αzz-αxx in the molecular axis system, where z is the symmetry axis; Φ is the hexadecapole moment; E is the dipole-octopole polarizability, which has a second-rank spherical tensor component E2 and a fourth-rank component E4. The van der Waals dispersion dipole is given by an integral over imaginary frequencies, where the integrand is a product of the polarizability at imaginary frequency α(iω) and the dipole-dipole-quadrupole hyperpolarizability B(0,iω). The B tensor is a fourth-rank Cartesian tensor with spherical-tensor components of ranks 0, 2, and 4.Table 2
Long-range dipole induction mechanisms that contribute to the coefficientsAλλ'ΛL of (2) for a pair of molecules A and B [17, 18].
Induction mechanismPower lawPropertiesCoefficientsQuadrupolar fieldR-4Θ,α̅A2023, A0223Θ,α∥-α⊥A22Λ3, Λ=2,3,4Hexadecapolar fieldR-6Φ, α̅A4045, A0445Φ, α∥-α⊥A42Λ5, Λ=4,5,6A24Λ5, Λ=4,5,6Nonuniform field gradientR-6Θ,E2A2245Θ,E4A42Λ5, Λ=4,5,6A24Λ5, Λ=4,5,6Back-inductionR-7Θ,α̅, α∥-α⊥A0001A2021, A0221A2023, A0223A2221A22Λ3, Λ=2,3,4A2245A4043, A0443Θ,α∥-α⊥A2021, A0221A2023, A0223A22Λ1, Λ=0,1,2A22Λ3, Λ=2,3,4A2245A4221, A2421A42Λ3, Λ=2,3,4A24Λ3, Λ=2,3,4A42Λ5, Λ=4,5,6A24Λ5, Λ=4,5,6DispersionR-7α̅ (iω), B0(0, iω)A0001α̅ (iω), B2(0, iω)A2021, A0221A2023, A0223α∥(iω)-α⊥(iω), B0(0, iω)A2021, A0221A2023, A0223α̅(iω), B4(0, iω)A4043, A0443α∥(iω)-α⊥(iω), B2(0, iω)A22Λ1, Λ=0,1,2A22Λ3, Λ=2,3,4A2245α∥(iω)-α⊥(iω), B4(0, iω)A4221, A2421A42Λ3, Λ=2,3,4A24Λ3, Λ=2,3,4A42Λ5, Λ=4,5,6A24Λ5, Λ=4,5,6For distinct molecules 1 and 2, or for chemically identical molecules that have different bond lengths, all of the dipole coefficients listed in Table2 are nonzero, although some of the coefficients may be quite small numerically. For chemically identical molecules, when r1=r2, the coefficients A0001, A22Λ1 with Λ≠1, A22Λ3 with Λ≠3, and A2245 vanish; the remainder are nonzero. The coefficients A0λλL and A24ΛL can be obtained from the coefficients Aλ0λL and A42ΛL via the relations(4)A0λλL=-𝒫12Aλ0λL,A24ΛL=(-1)Λ+1𝒫12A42ΛL,
where 𝒫12 interchanges the labels of molecules 1 and 2. For centrosymmetric molecules such as H2, the dipole coefficients Aλλ′ΛL vanish unless λ and λ′ are both even. Also, due to the Clebsch-Gordan coefficients in (2), nonvanishing contributions are found only if Λ=L-1, L, or L+1. Coefficients with higher values of λ and λ′ than those listed are of higher order than R-7 at long-range, although they may represent significant short-range overlap effects.From the dipole values in Table1, we have obtained a set of A coefficients by least-squares fit (at each R value) to (2), for r1=r2=1.449 a.u. From the fit, we have been able to determine the coefficients A2021, A0221, A2023, A0223, A2211, A2233, A4043, A0443, A4045, A0445, A4221, A2421, A4223, A2423, A4233, A2433, A4243, A2443, A4245, A2445, A4255, A2455, A4265, A2465, A4267, and A2467. We have kept all of these coefficients, as well as A0001 and A2201, in the calculations with unequal bond lengths for molecules 1 and 2. However, for R≥4.0 a.u and r1=r2=1.449 a.u., the least squares fit shows that the first ten coefficients are numerically important, while the remaining coefficients are essentially negligible. At R=4.0 a.u., the remaining coefficients do not exceed 7.0·10-5 a.u. in absolute value, and the values drop off rapidly with increasing R. Table 3 gives our results for A2021, A2023, A2211, A2233, A4043, and A4045; the other numerically significant coefficients are given by the relations A0221 = -A2021, A0223 = -A2023, A0443 = -A4043, and A0445= -A4045.Table 3
Dipole expansion coefficientsAλλ'ΛL(in a.u., multiplied by 106) for H2–H2 with r1=r2=1.449 a.u. Results from this calculation, compared with results of Meyer et al. [13] (MBF), Fu et al. [22] (FZB), long-range results [17, 18] through order R−7(LR), and quadrupole-induced dipole coefficients (QID).
R (a.u.)4.05.06.07.08.09.010.0A2021This work99832123407731342MBF10401219042984207—FZB10385218442783196—LR27959166210A2023This work−20065−8076−3725−1950−1124−695−455MBF−19967−7953−3688−1939−1119−692—FZB−19949−7946−3685−1938−1118−692—LR−19687−7652−3603−1921−1118−695−455QID−17628−7221−3482−1880−1102−688−451A2211This work40286183000MBF3327414200—FZB3327414200—LR−41−9−2−1000A2233This work202097751428917110770MBF1992949498280166104—FZB1991949498279166104—LR2588108853028816910670QID2726111753829117010670A4043This work690180429200LR20443124210A4045This work−845−283−97−37−16−8−4MBF−1523−450−135−47−19−9—FZB−1517−447−134−46−19−9—LR−1040−273−91−36−16−8−4In Table3, the results are also compared with results from two earlier ab initio calculations of the H2–H2 dipole with r1=r2=1.449 a.u., reported by Meyer et al. [12], Meyer et al. [13], and Fu et al. [22]. (The signs in Table 3 follow from our choice of the positive direction of the intermolecular vector R.) Meyer et al. [12, 13] used configuration-interaction wave functions including single, double, and triple excitations from a reference Slater determinant, in a (7s 1p) basis of Gaussian primitives on each H center, contracted to [3s 1p] and augmented by a (3s, 2p, 2d) basis at the center of the H–H bond, giving a total of 31 basis functions for H2 [11]. They performed calculations for 18 relative orientations that provided 9 nonredundant Cartesian dipole components. Fu et al. [22] employed the same basis to generate the CCSD (T) wave functions, in calculations for H2–H2 in 13 relative orientations, selected so that μY=0 in all cases. To find the dipoles, they used finite-field methods, with two fields that were equal in magnitude but opposite in sign. From Table 3, it is apparent that the results of Fu et al. (FZB) [22] agree well with the earlier results given by Meyer et al. (MBF) [13].For the largest coefficients,A2023 and A0223, our results are in excellent agreement with both of the earlier calculations: The percent differences between our results and those of Meyer et al. [13] are largest at R=5.0 a.u. (1.52%) and R=6.0 a.u. (0.99%); the remaining differences in these two coefficients average to 0.48%. We have obtained results at R=10.0 a.u., which were not given previously. The differences between our values for A2233 and those of Meyer et al. [13] are typically ~3% (smaller at R=4.0 a.u.). Differences in the values of A2021 and A0221 are ~5% or less at short range (R≤6.0 a.u.), where these coefficients have their largest values. At longer range, the absolute discrepancies are smaller, although the differences are larger on a relative basis. The principal differences in the dipole coefficients are attributable to the inclusion of A4043 and A0443 in our work; this affects the values of A4045, A0445, and A2211 (to a lesser extent).In Table3, the ab initiovalues of the coefficients are also compared with values based on the quadrupole-induced dipole model (QID) and the long range model (LR), which is complete through order R-7. The LR calculations include hexadecapolar induction, back-induction, and van der Waals dispersion effects, in addition to quadrupolar induction. The QID and LR calculations are based on the value of the H2 quadrupole computed by Poll and Wolniewicz [23], the value of Θ interpolated to r=1.449 a.u. given by Visser et al. [24], the hexadecapole computed by Karl et al. [25], the polarizabilities and E-tensor values given by Bishop and Pipin [26], and the dispersion dipoles computed from the polarizability and dipole-dipole-quadrupole polarizability at imaginary frequencies, also given by Bishop and Pipin [27].The coefficientA2023 depends primarily on the quadrupole-induced dipole: the difference between the QID approximation and our result is ~12% at R=4.0 a.u., ~10.6% at R=5.0 a.u., ~6.5% for R=6.0 a.u., and smaller at larger R. The QID model gives remarkably good values for this coefficient, even when R is quite small. Agreement with the full long-range model is somewhat better, with errors of ~5.25% at R=5.0 a.u. and only 1.88% at R=4.0 a.u. Quadrupole-induced dipole effects are also present in the coefficient A2233; this coefficient fits the QID and LR models quite well for R≥6.0 a.u., but the percent errors in these approximations are larger than those in A2023 for R=4.0 and 5.0 a.u. It should be noted that the back-induction and dispersion contributions have the same sign in A2023 but opposite signs in A2233.At long range the values ofA4045 and A0445 depend on hexadecapolar induction, which varies as R-6; there are no other contributions through order R-7. We find strong agreement between the values of these coefficients and the hexadecapole-induced dipole terms (which determine LR), for R≥5.0 a.u.; short-range effects become significant when R is reduced to 4.0 a.u. In contrast, A2021, A0221, A2211, A4043, and A0443 seem to reflect the short-range exchange, overlap, and orbital distortion effects predominantly. For these coefficients, the leading long-range terms of back-induction and dispersion vary as R-7; and they contribute with opposite signs in each case, further reducing the net effect of the long-range polarization mechanisms, in these particular dipole coefficients.As noted above, we have carried out calculations with 28 different combinations of bond lengths in molecules 1 and 2.Ab initio calculations have been completed for pairs with each bond length combination, in each of the 17 relative orientations, at each of 7 separations between the centers of mass, and for at least six values of the applied field in the X, Y, or Z direction.In the work of Meyer et al. on the absorption spectra ofH2–H2 pairs in the fundamental band, results for the Cartesian components of the pair dipoles are listed for four nonredundant pairs of bond lengths, (ro, ro), (ro, r-), (ro, r+), and (r-, r+), with ro=1.449 a.u., r_=1.111 a.u., and r+=1.787 a.u. [13]. Fu et al. [22] augmented this set by the addition of a larger bond length, r++=2.150 a.u., and reported results for all ten nonredundant pairs of configurations with the bond lengths drawn from the set {ro,r_,r+,r++}. In the current work, we have included ro, three bond lengths smaller than ro (1.280 a.u., 1.111 a.u., and 0.942 a.u.), and four bond lengths larger than ro (1.787 a.u., 2.125 a.u., 2.463 a.u., and 2.801 a.u.), in order to examine new portions of the dipole surface, particularly those that may become significant for photon absorption at higher temperatures. The specific nonredundant length combinations used in the calculations are (r1,r2) = (2.801, 2.125), (2.801, 1.787), (2.801, 1.449), (2.801, 1.280), (2.801, 1.111), (2.801, 0.942), (2.463, 2.125), (2.463, 1.787), (2.463, 1.449), (2.463, 1.280), (2.463, 1.111), (2.463, 0.942), (2.125, 1.787), (2.125, 1.449), (2.125, 1.280), (2.125, 1.111), (2.125, 0.942), (1.787, 1.449), (1.787, 1.280), (1.787, 1.111), (1.787, 0.942), (1.449, 1.449), (1.449, 1.280), (1.449, 1.111), (1.449, 0.942), (1.280, 1.111), (1.280, 0.942), and (1.111, 0.942), with all bond lengths in a.u.To illustrate the results for pairs with one or both bond lengths displaced fromro (the averaged internuclear separation in the ground vibrational state of H2), in Table 4 we list our values for the dipole expansion coefficients when r1 = 1.787 a.u. and r2 = 1.449 a.u., and we compare with the values given earlier by Fu et al. [22]. In general, we find excellent agreement. The values of A0001, A2021, A0221, A2023, A0223, A2233, A2243, and A2245 agree quite closely, particularly given the extension of the basis set and the corrections for hyperpolarization effects included in the current work. A few of the coefficients show larger differences, based on differences in the fitting procedures. In the current work, we have omitted the coefficients A2221 and A2223, which were included by Fu et al.; this contributes to the difference in the fitted values of A2211. On the other hand, we have included A4043 and A0443, which were omitted by Fu et al. [22]; this probably accounts for the difference in the values of A4045 and A0445 shown in Table 4. Our inclusion of A4221, A2421, A4223, A2423, A4243, A2443, A4245, A2445, A4265, A2465, A4267, and A2467 in the fitting procedure also causes slight shifts in the values of the other coefficients.Table 4
Dipole expansion coefficientsAλλ'ΛL (in a.u., multiplied by 106) for H2–H2 with r1=1.787 a.u. and r2=1.449 a.u. The results from this calculation are compared with the results of Fu et al. (FZB), [22].
R (a.u.)4.05.06.07.08.09.010.0A0001This work−22960−5786−1241−203−10139FZB−21869−5518−1232−231−296—A2021This work2065346189281682621FZB2129046889631944515—A0221This work−10595−2394−486−95−16−1−2FZB−11028−2450−508−108−28−10—A2023This work−32456−12335−5392−2735−1554−957−624FZB−32287−12113−5368−2749−1568−966—A0223This work2391610071476425251459905591FZB237789865468524881439889—A2211This work7331663770−20FZB52812626300—A2233This work30791497789443260161106FZB29521415750423253158—A2243This work−316−242−150−88−55−34−23FZB−375−263−148−83−49−30—A2245This work416180863919105FZB4331847229136—A4043This work198152912329831A0443This work−623−185−43−9010A4045This work−2079−684−224−84−35−16−9FZB−3956−1129−322−108−43−20—A0445This work9893641315323125FZB1559524169612511—No previous results are available for comparison when one or both of the molecules in the pair have bond lengths of 0.942 a.u., 1.280 a.u., 2.125 a.u, 2.463 a.u., or 2.801 a.u. In Table5, we provide results for one such combination of bond lengths, with r1=2.463 a.u. and r2=1.787 a.u. The coefficients listed in the top line of each set (and the corresponding coefficients for other pairs of bond lengths) were used in generating the rototranslational and vibrational spectra. These were obtained from fits that included 26 dipole coefficients all together (with A2211 and A2233, but not A2221 and A2223); immediately below those results in each set, we list values obtained from fits with 27 dipole coefficients (including A2221 and A2223, but not A2211). We find that the coefficients A0001, A2021, A2023, A2243, A2245, A4043, and A4045 are numerically “robust;” these coefficients are little affected by the difference in the fitting procedure. The coefficients A0221, A0223, A2233, A0443, and A0445 show greater sensitivity, although the agreement tends to improve as the separation between the molecular centers R increases (particularly for A0223 and A2233). The full results for the new potential energy surface and the pair dipoles, with individual H2 bond lengths ranging from 0.942 a.u. to 2.801 a.u., will be reported and analyzed in a subsequent paper. However, here we note that the coefficients A2023, A0223, A2233, A4045, and A0445 appear to be dominated by long-range induction mechanisms, specifically quadrupolar induction for A2023, A0223, and A2233, hexadecapolar induction for A4045 and A0445, and E-tensor induction for A2245. When the logarithms of the absolute values of these coefficients are plotted versus the logarithms of the separations R between the molecular centers of mass, over the range from 8.0 a.u. to 10.0 a.u., the slopes are -4.20 for A2023, -4.08 for A0223, and -3.995 for A2233, all close to the quadrupolar-induction value of -4. Similarly, the slopes are -6.42 for A4045 and -6.34 for A0445, close to the value of -6 for hexadecapolar induction; and the slope is -5.84 for A2245, close to the value of -6 for E-tensor induction [17].Table 5
Dipole expansion coefficientsAλλ'ΛL (in a.u., multiplied by 106) for H2−H2 with r1 = 2.463 a.u. and r2 = 1.787 a.u.  Results from the fit used to calculate the spectra (top line in each set) are compared with an alternate fit, which includes A2221 and A2223, but not A2211.
R (a.u.)4.05.06.07.08.09.010.0A0001−67727−18429−4357−863−119814−65778−19365−4616−960−1122225A20216127816009362073412612−361859157303543705128160.5A0221−23618−6192−1472−322−60−9−1−14285−4351−1033−235−47−10−1A2023−77947−28920−11520−5413−2954−1786−1157−78659−28577−11425−5378−2956−1791−1161A02234748520508974151192948181911855386622606102655251295518091178A22112759674164374−2−1A22219063937201101687A2223380324946421871−19−15A22336468357119781144678424278−2544136914411024666430282A2243−1604−788−575−362−223−143−94−1711−737−561−357−223−143−95A2245410211395452661296635422210825302601296736A404385122914742175451458404296675618144134A0443−1310−616−178−41−801−1531−510−149−30−8−2−1A4045−7655−3106−984−339−134−62−32−7534−3165−1000−345−133−61−31A0445239310864151627033172640968382150713519


## 3. About the Spectra

The absorption spectrum is a quasicontinuum, consisting of many thousand highly diffuse, unresolved “lines,” corresponding to rotovibrational transitions from an initial state{ν1,j1,ν2,j2}, to a final state {ν1′,j1′,ν2′,j2′}, of the binary collision complex. Under the conditions encountered in cool stellar atmospheres, vibrational quantum numbers ν from 0 to about 5 occur with significant population numbers, with rotational quantum numbers j up to 20 or so, for H2 molecules.The isotropic potential approximation (IPA), which neglects the anisotropic terms of the intermolecular potential, is used for the calculation of the spectra [2]. Each “line” requires as input the matrix elements of the spherical dipole components [2](5)〈ν1j1ν2j2∣Aλ1λ2ΛL(R,r1,r2)∣ν1′j1′ν2′j2′〉,
and the isotropic component of the intermolecular potential for the initial (unprimed) state(6)〈ν1j1ν2j2∣V000(R,r1,r2)∣ν1j1ν2j2〉;
the potential for the final state is given by a similar expression, where all rotovibrational quantum numbers are primed. The line shape calculations proceed with these expressions as described elsewhere [2]. In (5), (6), as above, R designates the intermolecular separation and r1, r2 the intramolecular separations. The indices λ1λ2ΛL are the expansion parameters of the spherical dipole components in (2).Figure1 shows the calculated absorption coefficient α(ν;T), normalized by the numerical density ρ squared, at the temperature T of 297.5 K, and frequencies ν from 0 to 3000 cm-1 (the “rototranslational band”). Laboratory measurements [28] are shown for comparison (∙). Good agreement of theory and measurements is observed.Figure 1
The calculated absorption spectrum of pairs of molecular hydrogen in the rototranslational band ofH2, at the temperature of 297.5 K, and comparison with laboratory measurements (• from [28]).We note that similarly good agreement of theory and measurement was previously observed, based on an earlierab initio ID surface and a refined intermolecular potential [2, 12]. In the present work, a more complete induced dipole surface has been obtained and used, although the extension has not significantly affected the rototranslational band, shown in Figure 1. Additionally, a new potential energy surface has been obtained and used in the current work. This new potential surface (as well as the new ID surface) accounts for highly rotovibrationally excited H2 mole-cules; and the new surfaces will be essential for our high-temperature opacity calculations—but again, the extensions of the potential surface are of little consequence for the rototranslational band, Figure 1, near room temperature.The new potential surface is believed to be accurate in the repulsive region of the interaction, but it is not as extensively modeled in the well region, and at long range (dispersion part). Nevertheless, the measurements of the absorption spectra are as closely reproduced by the newab initio input, Figure 1, as they are by the earlier advanced models. Apparently, the collision-induced absorption spectra arise mainly through interactions in the repulsive part of the potential, which is certainly consistent with previous observations [2].The new opacity calculations of the fundamental andH2 overtone bands [29] show similar agreement with measurements. Figure 2 shows the calculated normalized absorption coefficients over a frequency band ranging from the microwave region of the spectrum to the visible. In these calculations, we have used the exact equilibrium populations for the initial states, which at 2000 K consist of v=0, 1, and 2, with many different rotational states, including highly excited states. For the final states (after a photon of energy up to 2.5 eV has been absorbed), we have included much higher rotovibrational states of the molecules. We have accounted for all of these states rigorously, using the new intermolecular potential and induced dipole surfaces.Figure 2
Calculated absorption spectrum of pairs of molecular hydrogen, from the far infrared to the visible, at the temperatures of 600 K (dashes), 1000 K (solid line), and 2000 K (dotted).The coarse structures seen in the spectrum correspond roughly to the rototranslational band (peak near 600cm-1), the fundamental band of H2 (peak near 4200 cm-1), and the first through fourth overtone bands of H2 (remaining peaks). Unfortunately, no measurements exist for these high-frequency data, but we feel that the results shown are of comparable reliability to the results in Figure 1.Calculations of the type shown supplement previous estimates, especially at the highest frequencies [10, 30]. Presently, we are attempting calculations of H2–H2 opacities at still higher temperatures (up to 7000 K). Moreover, similar calculations are planned for H2−He and H2−H collisional complexes.


## 4. Conclusion

We report opacity calculations of collisionalH2–H2 complexes for temperatures of thousands of kelvin and a frequency range from the microwave to the visible regions of the electromagnetic spectrum. The calculations are based on new ab initio induced dipole and potential energy surfaces of rotovibrating H2 molecules, and are intended to facilitate modeling the atmospheres of cool stars. Agreement with earlier theoretical work and laboratory measurements, where these exist, is excellent.



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*Source: 371201-2009-10-19.xml*