MOPAC Calculations within GAMESS
 
    Parts of MOPAC 6.0 have been included in GAMESS giving 
access to four semiempirical wavefunctions:  MNDO, AM1, 
PM3, and RM1.  RM1 is the most recent parameterization, 
replacing AM1 data for H, C-F, P-Cl, Br, and I.  See G. 
Bruno Rocha, R. Oliveira Freire, A. Mayall Simas, and 
J.J.P.Stewart, J.Comput.Chem. 27, 1101-1111(2006).

    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 survive 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 these calculations.  
All you need to do is specify GBASIS=MNDO, AM1, PM3, or RM1 
in the $BASIS group.  Note that this not only selects a 
particular "Hamiltonian" (parameter set), it also picks a 
Slater Type Orbital (STO) basis.

    MOPAC parameters exist for the following elements.  The 
printout when you run will give you specific references for 
each kind of atom.  The quote on alkali's below 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     Li Be         *  C  N  O  F
Na Mg        Al Si  P  S Cl     Na Mg        Al Si  P  S Cl
 K Ca ... Zn  * Ge  *  * Br      K Ca ... Zn Ga Ge As Se Br
Rb' * ...  *  * Sn  *  *  I     Rb' * ... Cd In Sn Sb Te  I
*   * ... Hg  *  *  *           *   * ... Hg Tl Pb Bi

         For RM1:
H
                  C  N  O  F
                     P  S Cl
                          Br
                           I
RM1 uses AM1 parameters for any element not listed above.

                         * * * * *

    MOPAC will not work with every option in GAMESS: the 
semiempirical wavefunctions must be RHF, UHF, and ROHF in 
any combination with run types ENERGY, GRADIENT, OPTIMIZE, 
SADPOINT, HESSIAN, and IRC.  Note that nuclear hessian runs 
use numerical finite differencing of analytic gradients.  
MOPAC's CI and half electron methods are not supported.
 
    Because the majority of the implementation is GAMESS 
rather than MOPAC6, you will notice a few improvements. 
Dynamic memory allocation is used, so 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, which has been taught about s 
and p STO basis sets.
 
    However, because of the STO basis set used in semi- 
empirical runs, the various property calculations coded for 
Gaussian (GTO) basis sets are unavailable.  This means 
$ELMOM, $ELPOT, etc. properties are unavailable.  Note that 
MOPAC6 did not include d STO integrals, so it is quite 
impossible to run transition metals.

    The PCM solvation model implemented in GAMESS can be 
used with MOPAC runs by GAMESS.
 
    To reduce CPU time, by default only the EXTRAP 
convergence accelerator is used by the SCF procedures.  For 
difficult cases, the DIIS, RSTRCT, and/or SHIFT options 
will work, but may add significantly to the run time.  With 
the Huckel guess procedure from GAMESS, most calculations 
will converge acceptably without these special options.

    The MOPAC implementation is able to run in parallel.

    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 MOPAC is "how well is my system modeled by an 
ab initio minimal basis set, such as STO-3G"?  If the 
answer is "not very well", there is a good chance that a 
semiempirical description is equally poor.