Programs GAUSSIAN

Unrestricted calculations often incorporate a spin annihilation step, which removes a large percentage of the spin contamination from the wave function. This helps minimize spin contamination but does not completely prevent it. The final value of (,S y is always the best check on the amount of spin contamination present. In the Gaussian program, the option iop(5/14=2) tells the program to use the annihilated wave function to produce the population analysis. 

The Gaussian program contains a hierarchy of procedures corresponding to different approximation methods (commonly referred to as different levels of theory). Theoretical descriptions for each of them may be found in Appendix A. The ones we ll be concerned with most often in this work are listed in the following table ... 

Due is a certain non-uniqueness in the symmetry assignments in D2h symmetry, some authors, e.g. in ref [9], instead designate the compressed ground state as B2g and the elongated one as ig. In the present paper, we follow the convention set by the Gaussian program... 

The optimized geometries are reported in Table 1. The total and relative energies of all species illustrated in Figure are presented in Table 2. All calculations have been carried out with the 82 and 90 versions of the GAUSSIAN program system [4]. 

Suppose one wanted to study a biological system, possibly as small as an amino acid or as big as a polypeptide or a small nucleotide strand. There are a number of computer programs equipped to handle such systems, including the Gaussian programs. 

The Gaussian programs are the product of Gaussian, Inc., Pittsburgh, Pennsylvania. They perform quantum chemical calculations, using either semi-empirical methods such as AMI, MINDO/3, PM3, or ab initio calculations which have been discussed previously. 

The calculations described in this chapter were performed using one of the Gaussian programs from Pople s group and the CADPAC program of Handy. The current versions of these programs calculate the second derivative of the energy analytically. 

Molecular Orbital Calculations. The most sophisticated and theoretically rigorous of the molecular orbital methods are ab initio calculations. These are performed with a particular mathematical function describing the shape of the atomic orbitals which combine to produce molecular orbitals. These functions, or basis sets, may be chosen based on a convenient mathematical form, or their ability to reproduce chemical properties. Commonly used basis sets are a compromise between these two extremes, but strict ab initio calculations use only these mathematical functions to describe electronic motion. Representative of ab initio methods is the series of GAUSSIAN programs from Carnegie-Mellon University (11). In general, these calculations are computationally quite intensive, and require a large amount of computer time even for relatively small molecules. 

The CBS methods are available as keywords in the Gaussian program. The CBS-Q, CBS-q, and CBS-4 methods are incorporated in Gaussian 9475 and later versions. The CBS-QB3 method is available in Gaussian 98."... 

A G3 calculation [179] as implemented in the Gaussian programs uses eight steps ... 

Since the development of the Onsager model, there have been a number of elaborations on the model [4,5]. For example, the spherical cavity has been replaced by molecularly-shaped cavities. The state of the art within the field of solvent effects described by continuum solvent models is now implemented in, e.g., the Gaussian program package. 

The third step is the VB calculation itself. Hereafter, we only consider the 14 valence electrons and the 26 basis functions that are kept for the definition of the VB orbitals, in the order 2S, 2PX, 2PY, 2PZ, 3S, 3PX, and so on, as in the gaussian program (see Table 10.1). There are eight VB orbitals, the definitions of which are specified in the section orb . In this section, the first line indicates the number of basis functions over which each VB orbital is being expanded. These basis functions are specified in the following lines, for each VB orbital. The VB orbital 1 can be expanded on the basis functions 1, 4, 5, 8, 9, that is, the 2S, 2PZ, 3S, 3PZ, and DO basis functions of the Fa atom. The second VB orbital can span the basis functions 2PX, 3PX and D+1 on Fa, it is therefore an orbital of tt type, and so on. Note that the numbering of the basis functions, in this step, corresponds to the basis functions that are kept for the VB calculations. This numbering is therefore different from the one that appears in the Hartree—Fock output. 

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