Development of a novel direct SCF-program for the Hartree Fock treatment of molecular systems with special emphasis on biomolecular ligand field and reaction field problems

The aim of the present work was to establish a new quantum chemistry direct SCF program, based on the Hartree Fock theory, for the evaluation of molecular orbitals. For a correct description of electron repulsion, we have made use of the recursive relations for ERI computation due to Obara and Saika for the calculation of ERIs (Electron Repulsion Integrals) which is the most serious problem. Direct SCF was performed, which means that for each of the individual iteration steps the total number of ERIs is calculated new but no storage, neither main-memory nor disk space of ERIs, needs to be done, which enables simulations of even very large molecules as well. Some partial matrices within the Hamiltonian were computed with the help of the transformation of cartesian Gaussian basis functions to Hermitian like functions according to McMurphie and Davidson. Special emphasis has been given to the possibility of coupling the new quantum mechanical tool with other methods, for example molecular dynamics, which means that a complex great protein for example could be regarded as ligand field to a small core region, prosthetic group, co-factor, etc. that should be treated quantummechanically to the full extent. Furthermore, a unique module has been developed, that will respect reactionfield effects arising from some possible Solvation Shell, which will result in a certain SCRF job, where we are currently able to include different correction terms for spherical cavities up to the hexadecapole moment. (author)