Before we start, we need a working definition for MOPAC. The following description has been used many times to describe MOPAC: MOPAC is a general-purpose, semiempirical molecular orbital program for the study of chemical reactions involving molecules, ions, and linear polymers. It implements the semiempirical Hamiltonians MNDO, AM 1, MINDO/3, and MNDOPM3, and combir_es the calculations of vibrational spectra, thermodynamic quantities, isotopic substitution effects, and force constants in a fully integrated program. Elements parameterized at the MNDO level include H, Li, Be, B, C, N, O, F, A1, Si, P, S, C1, Ge, Br, Sn, Hg, Pb, and I; at the PM3 level the elements H, C, N, O, F, A1, Si, P, S, C1, Br, and I are available. Within the electronic part of the calculation, molecular and localized orbitals, excited states up to sextets, chemical bond indices, charges, etc. are computed. Both intrinsic and dynamic reaction coordinates can be calculated. A transition-state location routine and two transition-state optimizing routines are available for studying chemical reactions.

MOPAC: A semiempirical molecular orbital program

The effect of charge-transfer doping on the geometric and electronic structures of conjugated polymers has been investigated at the ab initio level with explicit consideration of the doping agents. Three systems were chosen for study as prototypical examples of conjugated polymers with nondegenerate ground states: polyparaphenylene, polypyrrole, and polythiophene. As a result of charge transfer with electron-donating dopants, extra charges appear on the polymer chains and induce strong geometry modifications. The lattice evolves from an aromatic structure towards a quinoid-like structure. Charged defects associated with lattice deformations such as spinless bipolarons are formed. The influence on the electronic structure of the polymer chains is such that with respect to the undoped case, new states appear within the gap. For the maximum doping levels experimentally achieved, band-structure calculations demonstrate that the states in the gap overlap to form bipolaron bands, a few tenths of an electron volt wide. The presence of these bipolaron bands is consistent with optical data as well as with magnetic data which suggest that the charge carriers in the highly conducting regime are spinless.

Highly conducting polyparaphenylene, polypyrrole, and polythiophene chains: An ab initio study of the geometry and electronic-structure modifications upon doping

The approach to the extension of the optimized potentials for liquid simulations all-atom (OPLS-AA) force field, which was tested for pyridine and the diazenes in the first paper of this series, has been extended to pyrrole, pyrazole, imidazole, furan, isoxazole, and oxazole Standard OPLS Lennard-Jones parameters are used for the nonbonded interactions in conjunction with partial charges obtained from fitting to RHF/6-31G* electrostatic potential surfaces The harmonic bond stretching and angle bending terms are adopted mostly from the AMBER force field, although the addition of two new atom types was required The resultant force field was tested by computing the gas-phase structures of the heterocycles, heterocycle−water interaction energies, properties of the pure liquids, and the relative free energies of hydration for pyrrole and imidazole

Development of an All-Atom Force Field for Heterocycles. Properties of Liquid Pyrrole, Furan, Diazoles, and Oxazoles

The theory for computing analytical energy gradients for second-order multireference perturbation theory (MRPT2) with arbitrary multiconfiguration self-consistent field (MCSCF) reference functions is derived and implemented. MRPT2 with complete active space reference functions (CASPT2) is a special case. In our method the configurations with two electrons in the external orbital space are internally contracted. This ansatz strongly reduces the length of the configuration expansion as compared to uncontracted wave functions, but avoids bottlenecks occurring when fully contracted first-order wave functions are used. The method, which also allows the use of state-averaged MCSCF reference functions, is applied to optimize the structures of low-lying valence and Rydberg states of Pyrrole. Nonplanar equilibrium structures and large geometry relaxation effects on the excitation energies are found for the valence states. From the results it can be concluded that CASPT2 underestimates the excitation energies of the valence states, but leads to accurate results for the Rydberg states.

Analytical energy gradients for internally contracted second-order multireference perturbation theory

A reinvestigation and extension of the observed VUV and EEL spectrum of pyrrole has been carried out, and the spectra assigned by means of high level multi-reference multi-root CI studies. A similar reasssessment of the UV-photoelectron spectrum leads to the conclusion that our previous assignment of IP3 to ionisation of the inner π-level (1b1) is correct and that most other assignments to ionisation of a σ-electron are incorrect. This is supported by the calculation of a wide range of Rydberg states derived from the five least bound orbitals, in the IP order: 1a2−1 1 ππ ∗ states of symmetry 1A1 and 1B2. The range of calculated valence states of ππ ∗ -type has been extended to include those from 1b1.

The electronic states of pyrrole studied by optical (VUV) absorption, near-threshold electron energy-loss (EEL) spectroscopy and ab initio multi-reference configuration interaction calculations

Use Nygaard

Papers

Microwave spectra of isotopic pyrroles. Molecular structure, dipole moment, and 14N quadrupole coupling constants of pyrrole