MOPAC calculations within GAMESS

Parts of MOPAC 6.0 have been included in GAMESS so that the GAMESS user has access to three semiempirical wavefunctions: MNDO, AM1 and PM3. These wavefunctions are quantum mechanical in nature but neglect most two electron integrals, a deficiency that is (hopefully) compensated for by introduction of empirical parameters. The quantum mechanical nature of semiempirical theory makes it quite compatible with the ab initio methodology in GAMESS. As a result, very little of MOPAC 6.0 actually is incorporated into GAMESS. The part that did make it in is the code that evaluates

  1. the one- and two-electron integrals,
  2. the two-electron part of the Fock matrix,
  3. the cartesian energy derivatives, and
  4. the ZDO atomic charges and molecular dipole.

Everything else is actually GAMESS: coordinate input (including point group symmetry), the SCF convergence procedures, the matrix diagonalizer, the geometry searcher, the numerical hessian driver, and so on. Most of the output will look like an ab initio output.

It is extremely simple to perform one of these calculations. All you need to do is specify GBASIS=MNDO, AM1, or PM3 in the $BASIS group. Note that this not only picks a particular Slater orbital basis, it also selects a particular "hamiltonian", namely a particular parameter set.

MNDO, AM1, and PM3 will not work with every option in GAMESS. Currently the semiempirical wavefunctions support SCFTYP=RHF, UHF, and ROHF in any combination with RUNTYP=ENERGY, GRADIENT, OPTIMIZE, SADPOINT, HESSIAN, and IRC. Note that all hessian runs are numerical finite differencing. The MOPAC CI and half electron methods are not supported.

Because the majority of the implementation is GAMESS rather than MOPAC you will notice a few improvments. Dynamic memory allocation is used, so that GAMESS uses far less memory for a given size of molecule. The starting orbitals for SCF calculations are generated by a Huckel initial guess routine. Spin restricted (high spin) ROHF can be performed. Converged SCF orbitals will be labeled by their symmetry type. Numerical hessians will make use of point group symmetry, so that only the symmetry unique atoms need to be displaced. Infrared intensities will be calculated at the end of hessian runs. We have not at present used the block diagonalizer during intermediate SCF iterations, so that the run time for a single geometry point in GAMESS is usually longer than in MOPAC. However, the geometry optimizer in GAMESS can frequently optimize the structure in fewer steps than the procedure in MOPAC. Orbitals and hessians are punched out for convenient reuse in subsequent calculations. Your molecular orbitals can be drawn with the PLTORB graphics program.

To reduce CPU time, only the EXTRAP convergence accelerator is used by the SCF procdures. For difficult cases, the DIIS, RSTRCT, and/or SHIFT options will work, but may add significantly to the run time. With the Huckel guess, most calculations will converge acceptably without these special options.

MOPAC parameters exist for the following elements. The quote means that these elements are treated as "sparkles" rather than as atoms with genuine basis functions. For MNDO:

 
           H
          Li  *          B  C  N  O  F
          Na' *         Al Si  P  S Cl
           K' * ...  Zn  * Ge  *  * Br
          Rb' * ...   *  * Sn  *  *  I
          *   * ...  Hg  * Pb  *
 
                   For AM1:                         For PM3:
           H                               H
           *  *          B  C  N  O  F     *  Be         *  C  N  O  F
          Na' *         Al Si  P  S Cl    Na' Mg        Al Si  P  S Cl
           K' * ...  Zn  * Ge  *  * Br     K' * ...  Zn Ga Ge As Se Br
          Rb' * ...   *  * Sn  *  *  I    Rb' * ...  Cd In Sn Sb Te  I
          *   * ...  Hg  *  *  *          *   * ...  Hg Tl Pb Bi
 

Semiempirical calculations are very fast. One of the motives for the MOPAC implementation within GAMESS is to take advantage of this speed. Semiempirical models can rapidly provide reasonable starting geometries for ab initio optimizations. Semiempirical hessian matrices are obtained at virtually no computational cost, and may help dramatically with an ab initio geometry optimization. Simply use HESS=READ in $STATPT to use a MOPAC $HESS group in an ab initio run.

It is important to exercise caution as semiempirical methods can be dead wrong! The reasons for this are bad parameters (in certain chemical situations), and the underlying minimal basis set. A good question to ask before using MNDO, AM1 or PM3 is "how well is my system modeled with an ab initio minimal basis sets, such as STO-3G?" If the answer is "not very well" there is a good chance that a semiempirical description is equally poor.


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