Comparative study of semiempirical methods for calculating interactions between large molecules with an application to the actinomycin-Guanine complex

Abstract

A comparison of semiempirical variational and perturbation calculations of intermolecular interactions is presented. The results are compared with ab inifio RHFcakulations, ab inifio perturbation calculations, and with experiments for systems representing hydrogen bonding, electron donor-acceptor complexes, incipient chemical reactions, and T-T stacking interactions. The failure of CNDO supermolecule calculations is discussed. Various approaches to a perturbation scheme using CNDO wave functions are investigated and an optimal method is suggested. The calculation of the stacking interactions in the deoxyguanosine actinomycin D complex suggests that a dual charge-transfer complex is formed. The experimental observation of two inequivalent binding sites on the actinomycin chromophore is interpreted in terms of the calculated energetics of complex formation.