ORBITAL CONNECTIONS FOR PERTURBATION-DEPENDENT BASIS-SETS

The use of perturbation-dependent basis sets is analysed with emphasis on the connection between the basis sets at different values of the p erturbation strength. A particular connection, the natural connection, that minimizes the change of the basis set orbitals is devised and th e second quantization realization of this connection is introduced. It is shown that the natural connection is important for the efficient e valuation of molecular properties and for the physical interpretation of the terms entering the calculated properties. For example, in molec ular Hessian calculations the natural connection reduces the size of t he relaxation term, leading to faster convergence of the response equa tions. The physical separation of the terms also means that first-orde r non-adiabatic coupling matrix elements can be obtained in a very sim ple way from a molecular Hessian calculation.