Ivanov V. Linear models and multireference state specific coupled cluster theory of many particle correlations in molecules

New methods capable of the exact account of electron correlation effects in molecules have been developed in the frameworks of both semiempirical and nonempirical quantum chemistry. The new multireference coupled cluster approach takes into account single and double excitations from all the reference determinants (CASCCSD). Efficient semiempirical and ab initio FCI approaches based on "iterative Krylov space" have been developed and implemented as a program code. They were shown to work well in calculating molecular states and external perturbations. Also the new FCI method has been used to examine the pi-electron correlation effects in calculations of dispersion constants. The FCI pi-electron method has been adapted for taking media effects into account using Kirkwood model and frequency-dependent polarizabilities calculations have been carried out to illustrate its potential.