MOZYME

The keyword MOZYME replaces the standard SCF procedure with a localized molecular orbital (LMO) method.  MOZYME was developed to allow very large organic compounds, specifically enzymes, to be easily calculated. The time required for a SCF calculation increases approximately linearly with the size of the system, see literature on MOZYME.

Memory considerations:  With 1Gb of RAM, systems of up to 10,000 atoms can be run without paging.  Above about 11,000 atoms, paging becomes severe, and jobs take longer than necessary.  With 2Gb of RAM, systems of up to 18,500 atoms can be run without significant paging.  Above that, the interatomic distance array cannot be created.  For a system of 19,000 atoms, the interatomic distance array would use 4*19000^2 bytes, or 1.4Gb.  When MOZYME is used for large systems, the machine becomes very slow for other activities.

If polarizabilities are required, use STATIC.  If keyword POLAR is used, then STATIC will be used instead.

The total charge on the system must be correct. If it’s not, then an error message will be printed. If GEO-OK is present, then the charge supplied will be replaced by the correct charge, and the job run.

Limitations of MOZYME

• Only closed shell RHF calculations are allowed. This means that MOZYME calculations are limited to species in their ground state. Radicals and electronic excited states cannot be run.  Ions that are definitely open-shell, such as Cr(III), cannot be run. 
• The results are not so precise, so for runs that need high precision (such as FORCE calculations), MOZYME should not be used.
• For large systems, the recommended geometry optimizer is LBFGS. This is a modified BFGS optimizer designed to minimize memory usage, and is the default for systems of 2000 parameters or more. If LBFGS is not wanted for any reason, then use BFGS, although it uses a lot more memory. The default optimizer, EF, uses a large amount of memory, and should therefore not be used in optimizing the geometry of large systems. In addition, because it uses a matrix inversion, it becomes very time consuming for large systems.  
• Electrostatic Potentials cannot be calculated, that is ESP cannot be used, however the Parametric Molecular Electrostatic Potential,  PMEP, can be used.

Recommended use of MOZYME

MOZYME can be used for simple geometric calculations, such as geometry optimization, transition state location, and intrinsic and dynamic reaction coordinates, and for the calculation of polarizability. For these calculations, MOZYME can be run as a "stand alone" calculation. If a partial geometry optimization is run, then the use of keyword RAPID should be considered.

For calculation of vibrational frequencies, frequency-dependent polarizability, and electronic excited states, MOZYME should be run first, to optimize the geometry, then a conventional MOPAC calculation run.

Another effective strategy is to run a MOZYME job, followed by a MOPAC job, using the OLDGEO feature. When geometry optimizations are being run, a MOZYME job can be run for a time, then a MOPAC job run, using RESTART. That is, the RESTART function will work when a geometry optimization or transition state location calculation is run, regardless of the method used in generating the SCF.