ISSN:
0020-7608
Keywords:
Computational Chemistry and Molecular Modeling
;
Atomic, Molecular and Optical Physics
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
The accurate treatment of molecular excited states is based on a matrix solution of the electronic Schrödinger equation in a suitable AO basis which relies heavily on the existence of a good approximate description of a given eigenfunction prior to the solution of a secular equation. The manner in which one or more roots corresponding to excited states can be traced from a rather simple starting point to the final results obtained from a large-scale CI treatment is outlined. In such calculations it is very useful to have a measure of the energy-lowering capability of configurations which differ by at most a double excitation from a series of reference configurations which represent the dominant contributions to one or more electronic states. The efficient computation of the required Hamiltonian matrix elements for this purpose, as well as for the construction of the final secular equations, can be achieved through the use of the table CI method, which does not rely on any special relationship between configurations to complete its work. Employing CI expansions of length no greater than 10000-20000 terms is very advantageous for the calculation of special properties of the resulting wavefunctions, especially those of spin-dependent quantummechanical operators. With the help of perturbative corrections it is possible to obtain energy results with such methods which are accurate to within 0.01 hartree of the full CI limit [R. A. Phillips, R. J. Buenker, P. J. Bruna, and S. D. Peyerimhoff, Chem. Phys. 84, 11 (1984)], whose error is small compared to that inherent in the use of conventional double-zeta-plus-polarization AO basis sets, therefore making further reduction of the CI errors almost inconsequential. Numerous examples for the calculation of the energy and properties of molecular excited states are surveyed.
Additional Material:
3 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/qua.560290317
