# Presentation: 089353

## Columbus Program SystemforMolecular Electronic Structure

- Relativistic Quantum Chemistry Capabilities
- Russell M. Pitzer
- Department of Chemistry
- Ohio State University
- Work done in collaboration with
- 	R. Shepard, Argonne National Lab
- 	T. Mueller, Research Center Jülich
- 	W.C. Ermler, University of Memphis
- 	I. Shavitt, University of Illinois
- 	B. Bursten, Ohio State University

## Columbus Program SystemInteractions - Energies

- One-electron
-      Kinetic Energy of Electrons
-      Electron-Nucleus Coulomb Attraction
-      Spin-orbit Interaction
- Two-electron
-      Electron-Electron Coulomb Repulsion

## Columbus Program SystemFormulation

- One-electron basis functions (orbitals):
-      Atomic orbitals (AOs)  linear combinations  Molecular orbitals (MOs)
- Many-electron basis functions:
-      Linear combinations of products of MOs and spin functions chosen to be
-      antisymmetric (Pauli Principle) and eigenfunctions of electron spin 
-      Configuration State Functions (CSFs)
- Wavefunctions (solutions to Schrödinger Eq.):
-      Linear combinations of CSFs
-      (Large, sparse matrix diagonalization)  energies etc.

## Columbus ProgramsReal Symmetric Eigenvalue Problem

- Sparse matrix; dimensions 104 to 109
- Need only small number of lowest eigenvalues
- Matrix elements generated on the fly in computing matrix-vector products
- (Iterative) Davidson method

## Columbus ProgramsParallelization

- Extraction of eigenvalues requires almost all of the computer time
- Sub-blocks of matrix handled in parallel in forming matrix-vector products
- Global Arrays software used
- Load balancing by assigning tasks in decreasing order of length

## Timing of CI Iteration

**Programming**: T. Mueller,
- Research Center, Juelich
**Tuning and Benchmarking**:
- M. Minkoff and R. Shepard,
- Argonne National Lab

## Columbus Programs

- Expect to incorporate pseudopotentionals from W.C. Ermler and M. Marino (SciDAC).
- Effectively, large-core pseudopotentials with outer core flexible and simply coupled to valence electrons

## Columbus Programs Applications

- UO22+ in Cs2UO2Cl2
- 	X-ray spectra (O K-edge) in 500 eV range.
- 	Excitation to valence orbitals including 5g.
- UO2
- 	Characterized lowest 32 electronic states.
- 	Low-lying states thermally populated 
- 			electronic hot bands
- 	Comparison with spectra of M. Heaven group
- 	Antisymmetric stretch frequencies found to be 776 cm-1(Ar matrix), 915 cm-1 (Ne matrix) by L. Andrews group.  Calculations and gas-phase experiments give results close to the high value (several groups).  Ar matrix electronic spectra give same electronic ground state as in gas phase (Heaven group).

## Columbus ProgramsApplications

- CUO – Observed shifts in matrix-isolation stretching frequencies (68,195 cm-1) between Ne and Ar hosts suggest different electronic ground states; support with DFT calculations without SO (Li et al. 2002).  CASPT2 + SO calculations give no explanation (Roos et al. 2003).
- Er3+ doped into GaN – example of laser material for optical-fiber signals.  Transition is 4I15/2  4I13/2. Crystal field causes splittings and intensities.

## Atomic Self-Consistent-Field Program

- Original version by
- 	C.C.J. Roothaan and P.S. Bagus  1963 (assembler)
- Many later versions in fortran
- Mainly used to optimize AO basis sets
- Version now available with
- 	fortran 90 memory allocation
- 	improved integral formulas
- 	simple vectorization features
- 	generalization to angular momenta 0 to 24
- 	some states with two open shells of the same symmetry
- 	simplified open-shell energy coefficients
- Correlation-consistent basis sets for core potentials now available:  http://www.chemistry.osu.edu/~pitzer