The ATMOL program written by Vic Saunders started life on the Atlas at Manchester University before Vic Saunders moved to the Atlas Computer Laboratory. The original version, ATMOL1, ran on the Chilton Atlas as did ATMOL2. ATMOL2 was also released on the 360/195 after Mike Chiu, Vic Saunders and Martyn Guest had done the port. ATMOL3 was mainly supported on the 360/195.
The ATMOL Manual given here relates to the version ported to the Cyber 205. The ATMOL Manual for Atlas has so far not been recovered.
ATMOL2 consisted of several packages which could be run concurrently or independently.
This package computes and outputs onto a file the two electron integrals evaluated over the atomic orbital basis set. The integrals section tends to be time consuming, and two methods are available for reducing computing time. The first is the use of a SYMCEN directive which indicates local centres of symmetry in the molecular geometry, and enables two electron integrals, equal in absolute magnitude by symmetry, to be identified and evaluated only once. The second is the use of an accuracy factor which sets two threshold values for the two electron integral evaluations.
In this package, before the iterative procedure starts, the facility exists of redefining the basis set as linear combinations of the basis functions. This can reduce the time required for diagonalisation of the Fock matrix as the elements of the matrix are blocked according to the irreducible representation to which the molecular orbitals belong. The matrix is then composed of a set of submatrices which can be diagonalised separately.
The integrals evaluated in the integrals section are over atomic orbitals, and must be converted in the SCF section to integrals over molecular orbitals.
The initial Fock matrix can be set up in three ways:
The Fock matrix is then diagonalised by the Jacobi method which zeros the off diagonal elements one at a time.
The Manual for the Nijmegen version of the SCF Program can be found at http://www.theochem.ru.nl/~pwormer/atmol/scf.html.
Two open shell procedures are available with ATMOL2, an RHF procedure and an unrestricted Hartree Fock (UHF) procedure, the former using a single Fock Hamiltonian constructed from the doubly and singly occupied orbitals, and the latter using a double Hamiltonian procedure to represent electrons of opposite spin. In the RHF procedure, the elements of the Fock matrix can be evaluated in different ways depending on the results desired.
In open shell procedures it is often difficult to obtain convergence in the SCF section due to a swapping of doubly occupied molecular orbitals with singly occupied ones, and of singly occupied orbitals with virtual ones. The ATMOL2 RHF and UHF can force convergence by using level shifters which increase the energy separation between the doubly and singly occupied orbitals, and between the singly occupied and virtual molecular orbitals. This prevents large scale swapping of molecular orbitals and tends to force convergence.
In this section, each molecular orbital is analysed, and a basis function population analysis is given for each orbital. The atomic populations contributed by each molecular orbital are found by summation of the appropriate basis function populations, and are output under the heading group populations. The total group populations are then found by summation of the group populations and are then doubled as each molecular orbital is analysed as if it contained one electron.
