GAUSSIAN computational program

Chapter 1 gives a short description of ab initio methods, Hartree-Fock and post-Hartree-Fock, focusing on the Gaussian computer programs. Chapter 2 describes semi-empirical calculations and their applications to biological systems. Chapter 3 addresses itself to electrostatic properties of molecules, as determined by quantum-chemical methods. The density functional method is discussed in chapter 4. Chapter 5 compares theoretically obtained parameters to experimental data. 

Ab initio molecular orbital calculations (using the Gaussian 80 computer program) on the barrier to pseudorotation (for the furanose ring) of two model compounds, 2-deoxy-/ -D- /) cero-tetrofuranosylamine (781) and 2-deoxy-2-fluoro-)3-D-erythrofuranosylamine (782) were reported. Al-... 

A computer program for the theoretical determination of electric polarizabilities and hyperpolarizabilitieshas been implemented at the ab initio level using a computational scheme based on CHF perturbation theory [7-11]. Zero-order SCF, and first-and second-order CHF equations are solved to obtain the corresponding perturbed wavefunctions and density matrices, exploiting the entire molecular symmetry to reduce the number of matrix element which are to be stored in, and processed by, computer. Then a /j, and iap-iS tensors are evaluated. This method has been applied to evaluate the second hyperpolarizability of benzene using extended basis sets of Gaussian functions, see Sec. VI. 

An even more reliable method of calculation of the atomic charges is the electrostatic potential-derived charge, devised by Kollman. This method, available in the Gaussian-92 program, assigns point charges to fit the computed electrostatic potential at a number of points on or near the van der Waals surface. 

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 basis set used for calculations were the STO-3G, the 3-21G, and the 6-31G basis sets, as implemented by the Gaussian-88 computer program. The energies of interaction were computed by using the supermolecule approach where the sum of the energies of the subsystems are substracted from the energy of the complex. All the species were geometry optimized. 

The Hartree-Fock calculations used the 6-31G basis set as implemented by the Gaussian-86 computer program.55 Protonated and unprotonated species were calculated and the proton affinities were obtained as the difference between the energy of the protonated species and the energy of the unprotonated ones. In addition, 6-31G calculations of the energy, using the 6-31G obtained geometry were performed. 

The Gaussian 80 program l, as implemented on an IBM 370/3033 mainframe computer at the Notre Dame Computing Center, was used for most of the calculations. Calculations were also conducted with the Gaussian 86 program22a implemented on a Digital VaxStation 3200 computer. Geometric optimizations were performed with the minimal STO-3G basis set 3, 24 he split-valence... 

All computations are carried out at the CCSD(T) level of theory. Ah initio calculations are performed using the Gaussian 98 program. For bromine... 

---