Hartree-Fock molecular orbital theory forms the cornerstone of ab initio quantum chemistry. Until the advances in the accuracy of Kohn-Sham density functional theory in the 1990's, Hartree-Fock theory was the method of choice for obtaining results for large molecules without resorting to standard empirical or semiempirical approaches. Molecular properties obtained by Hatree-Fock theory are generally at least qualitatively correct, although they can be quantitatively poor in many instances.
PSI3 solves the Hatree-Fock equations in a basis of Gaussian
functions using an iterative, self-consistent-field (SCF) procedure. The
final molecular orbitals are those which minimize the energy,
subject to the electron configuration specified by the user (or
guessed by the program). The process is continued until the largest
change in an element of the density matrix drops below 10
, where
is an integer specified by the convergence keyword.
Of course the efficiency of the iterative procedure depends on the choice of initial guess. The cscf module will attempt to use previously obtained orbitals as a guess if they are available. This can be particularly advantageous when diffuse functions are present; in that case, it may be easiest to run the computation with a smaller basis and project those orbitals onto the larger basis by specifying the -chkptmos command-line argument or the chkpt_mos=true keyword in input when running the input program for the larger basis. If old MO's are not available, cscf uses a core Hamiltonian guess by default. The convergence of the SCF procedure is accelerated by Pulay's direct inversion of the iterative subspace (DIIS) approach, and it is possible to modify the behavior of the DIIS through various keywords, although this is seldom necessary.
It is important to point out that the SCF approach does not rigorously guarantee that the final orbitals actually correspond to a minimum in orbital space; at convergence, the only guarantee is that the gradient of the energy with respect to orbital rotations is zero: this could be a global minimum, a local minimum, or a saddle point in orbital rotation space. While this is not usually an issue (typically the lowest minimum consistent with the electron configuration is found), it can be a problem sometimes for radicals, diradicals, bond breaking, or unusual bonding situations. The stable module can be used to test for the stability of Hartree-Fock wave functions.
The most commonly used keywords are found below. More specialized keywords are available in the man pages.