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 and Modeling proteins
Although a job that uses the MOZYME technique should give results that are the same as conventional SCF calculations, in practice there are differences. Most of these differences are small, but in some jobs the differences between MOZYME and conventional SCF calculations, particularly the calculation of DHf, can be significant. A single point calculation using MOZYME and conventional methods would produce essentially the same DHf, and for the purposes of this discussion the results of a single SCF calculated by both methods can be regarded as being identical. The problem with different DHf occurs when multiple SCF calculations are performed, this is the situation in a geometry optimization or reaction path calculation. In such calculations, the LMOs that result from an SCF calculation are used as starting LMOs in the next SCF calculation. In the first SCF calculation, the starting LMOs are exact - they form rigorously orthonormal sets, one for the occupied and one for the virtual sets. At the end of the SCF, small errors arising from truncation of the LMOs and incomplete pairwise rotations give rise to a small degradation of the orthonormal nature of the LMOs. In a single SCF, this degradation is unimportant, but when many SCF calculations are done, the loss of orthonormality increases steadily. This manifests itself as an error in the calculated DHf and to a much smaller extent in the gradients, and therefore, by implication, in the geometry. The loss of orthonormality could be corrected by re-orthogonalizing the LMOs, but the CPU cost of this is great, and re-othogonalization is not done by default, although it can be done if desired using REORTHOG. Fortunately, a very simple procedure exists to completely correct this error: After any long run involving many MOZYME SCF calculations, use the final geometry generated as the starting point for a 1SCF calculation, and then use the DHf from that calculation. This strategy should be used:
(A) In global optimizations.
(B) In transition state location runs.
(C) At the end of IRC runs.
Do not use OLDENS as that would re-use the now-inaccurate sets of LMOs, and thus defeat the purpose of doing the 1SCF calculation. As mentioned above, the errors in the gradient are small, so the geometry is essentially unaffected by the loss of orthonormality. However, it is still a good practice to optimize geometries in three or more separate runs, if only to provide an opportunity to check that the calculation is proceeding as intended.
During geometry optimizations, the error in DHf caused by the deterioration of the LMOs can result in the energy rising near the end of the run. If this happens, the lowest energy structure will be output, instead of the last structure calculated.
By default, the M.O.s printed are LMOs. If canonical M.O.s are needed, use keyword EIGEN. EIGEN uses a large amount of memory and might not work if the system is too large. Even if it does work, it might take a lot of CPU time, so EIGEN should only be used with 1SCF.
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 itís not, the charge will be changed to that predicted by the Lewis structure. To prevent the charge being changed, use CHARGE=n, even in cases where the system is neutral, i.e., CHARGE=0. If CHARGE=n is not present, then the charge predicted by the Lewis structure will be used.
In some very specific cases, the MOZYME method can give rise to an incorrect SCF, specifically the MOZYME SCF solution corresponds to an electronic excited state. None of these cases ever occur in protein chemistry, unless severe errors are made, but users should be aware of their existence. The commonest incorrect SCF solutions are obtained when the system should not be represented by a Lewis structure. A much rarer type of error is found in some helical buckytubes when periodic boundary conditions are imposed. In cases of this type, the Lewis structure generator simply fails to make the correct Lewis structure, and instead attempts to correct a faulty structure by assigning charges to two or more carbon atoms. In all instances where incorrect SCF solutions are found, the correct solution can be obtained by the appropriate use of CVB or SETPI.
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.