MULT

MULT is a semi-empirical package for studying the electronic and geometrical
structure of organic molecules with well-defined "chemical bonds" written by
Andrei M. Tokmachev and Andrei L. Tchougreeff.

The method used in MULT stems from our earlier program sequence known as BF
(Boutleroff-Fock) which is based on the trial wave function taken as the
antisymmetrized product of strictly localized geminals. The atomic basis is
transformed to the hybride one by unitar transformations of one s- and three
p-orbitals for every heavy (non-hydrogen) atom. Two hybride orbitals are
uniquely assigned to every chemical bond. For every chemical bond the two-electron
wave function (geminal) with varying weights of two ionic and one covalent
configurations is contsructed. All the parameters of transformation from the
atomic basis to the hybride one and the weights of contributions to geminals
are determined on the ground of the variational principle. Later this simple
scheme has been extended by allowing various types of single-determinant wave
functions to be present either for extended π-systems in organic molecules or
in, so called, *close ligand shells* in the metal complexes (like those of
Zn(II)) or in hydrogen bonds. Next, the multipole approximation has been applied
to make feasible calculation of the subshell-dependent two-center electron-electron
interactions in the parameterizations like MNDO and higher of this family. The
package MULT calculates the optimal hybrid and molecular orbitals, the coefficients
of geminal expansions, and allows to determine the heats of formation of organic
molecules. The determination of the minimum on the potential energy surface is also
available. Due to geminal usage the method features *O(N)* dependence of the
required computational resourses on the system size.

Source: BF, by Andrei M. Tokmachev and Andrei L. Tchougreeff.

Documentation: A short **BF reference**
is available on-line. More elaborated version is under construction.