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.