MOZYME solver

MOZYME is a fast solver algorithm in MOPAC that accelerates the self-consistent field (SCF) cycle by representing the electronic ground state with localized molecular orbitals (LMOs) instead of canonical molecular orbitals. MOZYME is faster and uses less memory than the conventional solver algorithm in MOPAC once the system size exceeds a few hundred atoms. Formally, the solution time of the MOZYME solver scales linearly in the number of atoms if the number of SCF iterations is independent of system size, while the conventional solver scales cubically. However, MOZYME will not work for every system and cannot calculate properties of canonical molecular orbitals such as HOMO and LUMO energies. For a detailed description of the MOZYME solver, see its implementation paper.

The MOZYME solver is activated by adding the MOZYME keyword to a MOPAC input file. Many of MOPAC’s features are compatible with the MOZYME solver and require no further changes to the input file. However, MOZYME causes two important changes to a MOPAC calculation. First, a formal Lewis structure analysis is performed on the input geometry. Based on covalent bonding and shell-filling rules and inter-atomic distances, all atoms are assigned covalent bonds and a formal charge. Once this analysis is complete, MOZYME will try to set the total charge of the system to achieve a closed-shell electronic configuration. If it is unable to achieve a closed shell, MOZYME will end the calculation. Second, the total charge of a MOZYME calculation defaults to the total charge determined by the bonding analysis rather than a neutral charge. If a different total charge of x is required, then the CHARGE=x keyword also needs to be added to the input file. The MOZYME bonding analysis will then attempt to achieve a closed shell that is compatible with the total charge constraint.

The MOZYME solver needs the bonding analysis to generate a good initial guess for the LMOs. The LMOs are optimized using a sparse version of pseudodiagonalization, which usually needs a good initial guess to converge. If you believe that a system has a closed shell but MOZYME fails to detect it, then you can use the LEWIS keyword to provide more details of the bonding analysis for debugging purposes. See the online MOPAC manual for more information about the MOZYME solver and advanced keywords that can further guide and constrain its bonding and charge assignments.

The MOZYME solver is incompatible with vibrational and excited-state calculations, and polarizability calculations must be static rather than frequency-dependent. Because the MOZYME solver requires a closed shell, it is limited to restricted Hartree-Fock (RHF) calculations and cannot perform unrestricted Hartree-Fock (UHF) calculations.