Topic
AMPAC
About: AMPAC is a research topic. Over the lifetime, 37 publications have been published within this topic receiving 792 citations.
Papers
TL;DR: The solvent effects on the tautomeric equilibria were investigated by self‐consistent reaction field (SCRF) theory implemented in the AMPAC and MOPAC program in two different ways: one in which all the solvent relaxation is included in the quantum mechanics and the total energy must be corrected for the solvent change in energy, and a second in which the quantum Mechanics directly includes this term, method B.
Abstract: AM1 and PM3 semiempirical calculations are reported for the solvent effects on the tautomeric equilibria of 2-pyridone/2-hydroxypyridine and 4-pyridone/4-hydroxypyridine in the gas phase and solution. The solvent effects on the tautomeric equilibria were investigated by self-consistent reaction field (SCRF) theory implemented in the AMPAC and MOPAC program in two different ways: one in which all the solvent relaxation is included in the quantum mechanics and the total energy must be corrected for the solvent change in energy, method A; and a second in which the quantum mechanics directly includes this term, method B. The calculated (AM1, method A) tautomeric equilibrium constants (log K1) for 2-pyridone in the gas phase, cyclohexane, chloroform, and acetonitrile are −0.3, 0.3, 0.8, and 1.3, respectively, in good agreement with the experimental data (−0.4, 0.24, 0.78, and 2.17, respectively). For 4-pyridone/4-hydroxypyridine differences between calculated log K1 for the gas phase, chloroform and acetonitrile (−6.0, −2.6, and −1.2, respectively) and experimental data (< −1, 0.11, and 0.66, respectively) are larger but the experimental values are also less certain. The experimental acetonitrile data are disturbed by specific interactions. An extension of the SCRF for aqueous solutions is reviewed. © 1993 John Wiley & Sons, Inc.
140 citations
TL;DR: A set of FORTRAN subroutines for self-consistent reaction field solvation modeling in the framework of semi-empirical quantum-chemical methods is described and included on disk as discussed by the authors.
56 citations
TL;DR: In this article, the AM1, MNDO and MINDO/3 semi-empirical molecular orbital methods from the AMPAC computer program package were used to obtain the stable closed-shell 1 A g ground state.
Abstract: The N 20 molecule assumes a dodecahedral structure with I h group symmetry. Molecular orbital calculations were done using the AM1, MNDO and MINDO/3 semiempirical molecular orbital methods from the AMPAC computer program package. The stable closed-shell 1 A g ground state was obtained by the self-consistent field type calculation in all the above-mentioned methods. The optimized bond distance, first ionization potential, vibrational force constants and various thermodynamic properties were calculated for comparison. The evidence suggests that N 20 is a stable molecule.
26 citations
TL;DR: A gradient algorithm derived from a combination of a variable metric method and Pulay's direct inversion in the iterative subspace method for geometry optimization (GDIIS) is compared with theVariable metric method combined with an accurate linear search algorithm.
Abstract: Several improvements have been made to the gradient algorithms commonly used to optimize equilibrium and transition-state geometries at the semiempirical level. A gradient algorithm derived from a combination of a variable metric method (Davidon–Fletcher–Powell/Broyden–Fletcher–Goldfarb–Shanno) and Pulay's direct inversion in the iterative subspace method for geometry optimization (GDIIS) is compared with the variable metric method combined with an accurate linear search algorithm. The latter method is used routinely in the standard semiempirical program packages, MNDO, MOPAC, and AMPAC. The combined variable metric and GDIIS algorithm is also compared with GDIIS which uses a static metric. The performance of these algorithms is examined for a wide range of systems with respect to both choice of coordinate system (for cyclic molecules) and guess for the initial Hessian. The results show that the GDIIS method is up to ca. 40% more efficient than the variable metric combined with accurate line search algorithm: however, the exact savings vary depending on the coordinate system and initial Hessian. For noncyclic systems, variable-metric GDIIS is usually equal or superior to static-metric GDIIS, and consistently performs ca. 30% more efficiently than the variable metric combined with accurate line search algorithm. For the optimization of cyclic molecules, an improved estimate of the initial Hessian has increased the efficiency by at least a factor of two. Greater efficiencies (usually >40%) are also obtained when static-metric GDIIS is used to refine the geometry after the initial application of a transition-state search based on the variable metric combined with line search algorithm. On the basis of these results, we recommend several changes to the algorithms as currently implemented in the standard semiempirical program packages.
24 citations
Related Topics (5)
Performance Metrics
| Year | Papers |
|---|---|
| 2018 | 1 |
| 2012 | 2 |
| 2010 | 1 |
| 2009 | 1 |
| 2007 | 1 |
| 2005 | 1 |