Ioannis N. Ragazos

An evaluation of three direct MC-SCF procedures

Abstract The direct MC-SCF approach for modest active spaces is evaluated for (a) extended basis sets on small molecules and (b) modest sets on extended systems. An algorithm that scales as n2aN4 (na = number of active orbitals, N = number of AO) using Fock matrices appears to be quite efficient until na gets to 8 where an algorithm that scales as naN4 using 3 4 transformed integrals begins to become competitive. The storage requirement for the J and K operators in the naN4 algorithm has been removed with the cost of an additional direct Fock matrix construction.

Optimization and characterization of the lowest energy point on a conical intersection using an MC-SCF Lagrangian

Abstract A method is presented, in the context of MC-SCF theory, for the optimization of the lowest energy point of a conical intersection of two potential energy surfaces that does not require the use of Berrys phase condition. The technique uses existing methodology of quantum chemistry (analytical gradients) and does not require the determination of the Lagrange multiplier. The significance of the two constraints is discussed and a numerical demonstration is given.

A Strategy for Modelling of Chemical Reactivity using MC-SCF and MM-VB Methods

The problem of modelling chemical reactivity for larger molecular systems is discussed. A strategy involving VB theory implemented via a Heisenberg Hamiltonian is described. The central features of such an approach are a) it is possible to combine the quantum mechanics with force field methods for the study of reactivity and b) it is compatable with being parametrized using the best state of the art ab-initio methods that are available. The utility of the method is illustrated on the photochemistry of a steroid.