GAUSSIAN
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Gaussian 09 is the latest in the Gaussian series of electronic structure programs. Gaussian is used by chemists, chemical engineers, biochemists, physicists and others for research in established and emerging areas of chemical interest.
Starting from the basic laws of quantum mechanics, Gaussian predicts the energies, molecular structures, and vibrational frequencies of molecular systems, along with numerous molecular properties derived from these basic computation types. It can be used to study molecules and reactions under a wide range of conditions, including both stable species and compounds which are difficult or impossible to observe experimentally such as short-lived intermediates and transition structures. This article introduces several of its new and enhanced features.
GAUSSIAN was 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.
Nearly everything you need to know can be found on the Gaussian's homepage, [1] and the on-line manual, [2]
REMEMBER: You MUST use the "p" keyword for extra-print in all your calculations.
NMR parameters, the chemical shift
The NMR keyword can be added at the command line. It predicts NMR shielding tensors and magnetic susceptibilities (HF, DFT & MP2 methods).
The GIAO method is used by default, and it is one of the recommended methods. Usually these calculations are performed with IGLO-II basis sets, altough other basis sets can also been used.
To obtain the chemical shifts, the Isotropic Magnetic shielding (ppm) obtained from the Gaussian output, should be referenced to that of TMS (tetra-methylsilane) at the same level. i.e. at b3lyp/6-31g* basis set it is 31.92 ppm.
δ(X) (ppm) = isotropic(TMS) – isotropic(X)
Bader's analysis
If you would like to analyse your systems by using parameters of Bader's analysis (AIM Atoms in Molecules). You could use gaussian to get a .wfn file. To get it it is necessary to write output=wfn at the command line and the name of the wfn file at the end (after general basis sets).example_bader
Afterwards you could use Xaim to locate critical points, or to plot the maps of density, laplacian, ... Prof. Carles Bo is one of the fathers of Xaim. Xaim is available on kimik and it can be download for free at
http://www.quimica.urv.es/XAIM/