Kazuo Kitaura

Derivatives of the approximated electrostatic potentials in unrestricted Hartree–Fock based on the fragment molecular orbital method and an application to polymer radicals

The analytic energy gradient for the point charge approximation of the embedding potential is derived in the framework of unrestricted Hartree–Fock based on the fragment molecular orbital method (FMO). For this goal, we derive the necessary coupled-perturbed unrestricted Hartree–Fock equations, describing the response terms arising from the use of embedding atomic charges in dimer calculations. By a comparison to numerical gradients and with the aid of molecular dynamics, we show that the gradients have a high accuracy. A speed-up of the factor 7.3 is obtained for the largest system, when approximated potentials are used relative to the exact two-electron embedding. We apply the FMO method to polymer radicals and show that it has satisfactory accuracy in reproducing the geometries and energies of polymer radical reactions.