GAUSSIAN: Difference between revisions
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* If you have one or more hydrogens "bonded" to two atoms (close to two atoms), the default parameters of PCM will not be able to compute the molecular cavity for them. There are two solutions for this problem: | * If you have one or more hydrogens "bonded" to two atoms (close to two atoms), the default parameters of PCM will not be able to compute the molecular cavity for them. There are two solutions for this problem: | ||
1. | 1. Use a different model to build up the cavity. Change the default UAO which does not explicitly consider hydrogens, by UHF, PAULING, BONDING.. which consider hydrogens explicitly. | ||
2. | 2. Use the keyword SPHEREONH = atom number of the hydrogen "bonded" to two atoms. | ||
* An odd tip about PCM: If your PCM calculations work fine for most of your systems but they give problems to compute the cavity for few of them, with no apparent reason: try to modify slightly your coordinates. | * An odd tip about PCM: If your PCM calculations work fine for most of your systems but they give problems to compute the cavity for few of them, with no apparent reason: try to modify slightly your coordinates. | ||
A very small change of the coordinates may be enough to eliminate the problem. A good way to do that is to round the coordinates (i.e. from 2.347895 to 2.348) or re-optimize the molecule (the coordinates you will get will be slightly different). Even though that sounds quite stupid some times it is enough for the program to work. | A very small change of the coordinates may be enough to eliminate the problem. A good way to do that is to round the coordinates (i.e. from 2.347895 to 2.348) or re-optimize the molecule (the coordinates you will get will be slightly different). Even though that sounds quite stupid some times it is enough for the program to work. | ||
Revision as of 09:40, 13 January 2009
GAUSSIAN is a computational chemistry software program, first written by John Pople and released in 1970. According to the manual, it can deal with different computational approaches: molecular mechanics (AMBER, UFF, DREIDING), semi-empirical calculations (AM1,PM3,CNDO,INDO,MINDO/3,MNDO), SCF methods (Restricted, Unrestricted, and Restricted Open-shell Hartree-Fock), Møller-Plesset perturbation theory, DFT methods (Hybrid functionals, exchange functionals and correlation functionals), ONIOM (QM/MM method), Complete Active Space (CAS) and Multi-Configurational Self-Consistent Field calculations, Coupled Cluster calculations, QCI methods and Quantum chemistry composite methods.
Links
http://www.gaussian.com/tech_top_level.htm
http://en.wikipedia.org/wiki/GAUSSIAN
ABOUT PCM (CPCM, IPCM,..) :
- If you have one or more hydrogens "bonded" to two atoms (close to two atoms), the default parameters of PCM will not be able to compute the molecular cavity for them. There are two solutions for this problem:
1. Use a different model to build up the cavity. Change the default UAO which does not explicitly consider hydrogens, by UHF, PAULING, BONDING.. which consider hydrogens explicitly. 2. Use the keyword SPHEREONH = atom number of the hydrogen "bonded" to two atoms.
- An odd tip about PCM: If your PCM calculations work fine for most of your systems but they give problems to compute the cavity for few of them, with no apparent reason: try to modify slightly your coordinates.
A very small change of the coordinates may be enough to eliminate the problem. A good way to do that is to round the coordinates (i.e. from 2.347895 to 2.348) or re-optimize the molecule (the coordinates you will get will be slightly different). Even though that sounds quite stupid some times it is enough for the program to work.