# SCIGRESS - Computational Tools - MO-G (MOPAC)

MO-G (previously MOPAC), the Molecular Orbital Package, developed by James J. P. Stewart, provides a choice of methods for computing electronic properties of molecules. Scigress provides access to MO-G methods through a Windows environment.

The SCIGRESS MO-G application determines both an optimum geometry and the electronic properties of molecules by solving the Schrödinger equation using the semi-empirical Hamiltonians AM1, PM3, PM5 and PM6, developed by M. J. S. Dewar and J. J. P. Stewart respectively. SCIGRESS also supports the older parameter sets, MNDO and MINDO/3, and the newer parameter set, MNDOd. In addition, SCIGRESS extends AM1 to AM1/d.

The molecular orbitals, heat of formation, and molecular geometry derivatives obtained are used to calculate vibrational spectra, molecular geometries, force constants, and other properties of molecules, radicals, and ions. These quantities are used to calculate reaction trajectories and to investigate chemical reactions by locating transition states.

MO-G can calculate the geometry, energetics and reaction profiles of molecules in their excited states. It can include solvent effects in these calculations. Excited state calculations are important in simulation of photochemical processes. MO-G calculates heats of formation, rather than the energy required to separate the molecule into isolated nuclei and electrons as in quantum-chemical methods such as Extended Hückel Theory and ZINDO.

Although MO-G uses many concepts from quantum theory, thermodynamics, and advanced mathematics, a detailed understanding of these areas is not necessary. MO-G was written and designed with non-theoretical chemists in mind.

## MO-G Functionality

Semi-empirical Hamiltonians: MNDO, MINDO/3, AM1, PM3, MNDO-d, and PM5.

Transition metals: Sc, Ti, Zr, V, Cr, Mo, Fe, Co, Ni, Pd, Pt, Cu, Ag, Zn, Cd, and Hg.

Optimization: MOPAC uses the very robust Baker's Eigenvector Following procedure as the default geometry optimizer. Other options include: Broyden-Fletcher-Goldfarb-Shanno (BFGS), Davidon-Fletcher-Powell (DFP), Sigma, and McIver-Komornicki. Mixed internal and Cartesian coordinate input is allowed.

SCF Procedures: Restricted Hartree-Fock, Unrestricted Hartree-Fock, SCF-CI.

Giant Molecules: MOZYME method for solving the SCF equations is implemented that scales linearly in time and memory usage with the size of the system. The electronic properties of systems with more than 20,000 atoms, including proteins, polymers, semiconductors, and crystals can now be calculated in minutes.

Solvent effects: the linear COSMO technique and the Tomasi method (MST).

Electric Fields: the effect of applied external electric fields can be modeled.

Electrostatic Potentials: MO-G embodies two electrostatic potential methods, the Wang-Ford Parametric Electrostatic Potential (PMEP) and Merz-Besler ESP methods.

Vibrations: the normal modes of vibration of ground state and transition state systems can be calculated, including the force constants and effective mass. Isotopic substitution effects can be modeled.

Thermodynamic quantities: partition functions, enthalpies, heat capacities, and entropies can be calculated for any temperature, or range of temperatures.

Quantum Molecular Dynamics: MO-G can perform dynamics calculations as a function of time at constant total energy, controlled reduction of kinetic energy (cooling or simulated annealing) and controlled increasing kinetic energy (heating).