Quantum chemical programs

Before 1980, force field and semiempircal methods (such as CNDO, MNDO, AMI, etc.) [1] were used exclusively to study sulfur-containing compounds due to the lack of computer resources and due to inefficient quantum-chemical programs. Unfortunately, these computational methods are rather hmit-ed in their reliability. The majority of the theoretical studies under this review utilized ab initio MO methods [2]. Not only ab initio MO theory is more reliable, but also it has the desirable feature of not relying on experimental parameters. As a consequence, ab initio MO methods are apphcable to any systems of interest, particularly for novel species and transition states. 

Here, n corresponds to the principal quantum number, the orbital exponent is termed and Ylm are the usual spherical harmonics that describe the angular part of the function. In fact as a rule of thumb one usually needs about three times as many GTO than STO functions to achieve a certain accuracy. Unfortunately, many-center integrals such as described in equations (7-16) and (7-18) are notoriously difficult to compute with STO basis sets since no analytical techniques are available and one has to resort to numerical methods. This explains why these functions, which were used in the early days of computational quantum chemistry, do not play any role in modem wave function based quantum chemical programs. Rather, in an attempt to have the cake and eat it too, one usually employs the so-called contracted GTO basis sets, in which several primitive Gaussian functions (typically between three and six and only seldom more than ten) as in equation (7-19) are combined in a fixed linear combination to give one contracted Gaussian function (CGF),... 

J. P. Chandler, Quantum Chemical Program Exchange Program No. 307 (SIMPLEX), 1965. 

A specialized MOPAC computer software package and, in particular, its PM3 quantum-chemical program has been successfully applied in calculations. The results of calculations have shown that both oxygen atoms form bonds with two more active carbon atoms of CP molecular cluster (so-called bridge model of adsorption). The total energy of system after a chemical adsorption at such active atoms is minimal. 

In the last decade, quantum-chemical investigations have become an integral part of modern chemical research. The appearance of chemistry as a purely experimental discipline has been changed by the development of electronic structure methods that are now widely used. This change became possible because contemporary quantum-chemical programs provide reliable data and important information about structures and reactivities of molecules and solids that complement results of experimental studies. Theoretical methods are now available for compounds of all elements of the periodic table, including heavy metals, as reliable procedures for the calculation of relativistic effects and efficient treatments of many-electron systems have been developed [1, 2] For transition metal (TM) compounds, accurate calculations of thermodynamic properties are of particularly great usefulness due to the sparsity of experimental data. 

Finally, more sophisticated quantum-chemical programs like WIEN95 or the well-known program GAUSSIAN98 allow the calculation of the electric field gradient and the quadrupole parameters, but involving more computer time. Results obtained on this basis for a number of systems are collected in Table 17 They compare quite well... 

Quantum Chemical Program Exchange (QCPE), Creative Arts Bldg. 181, Indiana University, 840 State Hwy. 46 Bypass, Boomington, IN 47405, USA... 

This formal analogy is very helpful, since in this way, a dielectric continuum can easily be implemented in a quantum chemical program as an addition to the Coulomb interactions. 

Since the derivative of the B operator is also readily available in quantum chemical programs, only the derivative of the A matrix needs to be coded for the calculation of analytic gradients. 

Table 3 Some Quantum Chemical Programs That Can Be Used for Thermochemical Calculations Program Capabilities" Web Site... 

Several versions of modified INDO (MINDO) that employ such a parameterization have been proposed. These include MINDO/1, MINDO/2, MINDO/2, and MINDO/3, only the last of which resulted in a quantum chemical program that was widely used. MINDO/3151 is parameterized for H, B, C, N, O, F, Si, P, S, and Cl, although certain combinations of these atoms are not parameterized. The MINDO/3 method is no longer heavily used because the parameterized NDDO methods are generally more accurate. 

Nowadays a wide variety of quantum-chemical programs are disposable, which permit to calculate with high accuracy the equilibrium geometry of the molecules and their energy of formation. Theoretical methods have been developed for analytical calculation of the first and second derivatives of energy [8,9], so that the force-constant matrix FHT and the harmonic frequencies can be extracted from the quantum-mechanical calculations. Since as a rule the molecular orbitals (MO) obtained by the quantum-mechanical methods are spread around the entire molecule, the corresponding quantum-mechanical force fields incorporate the important effects of the off-diagonal interactions. 

Several commercial quantum chemical programs like Gaussian (by Gaussian, Inc., www.gaussian.com) and Spartan (by Wavefunction, Inc., www.wavefun.com) are available for the solution of the electronic Schrodinger equation. 

Silvera-Goldman form [8] was used, which good describes experimental data of molecules H2 interaction including weak van der Waals H2 molecule interaction. Interaction potential between hydrogen molecule and carbon atoms was accounted in the same way. Because of importance of taking into account van der Waals interaction having correlational nature we could not use any quantum chemical programs for ab initio calculations because this interaction does not considered in them. 

As the method readily lends itself to implementation into a quantum chemical program, the authors also give the appropriate formulation for the CNDO method and report calculations on L-alanine and (-i-)-(3/ )-methylcyclohexanone, which, as they state, compare favorably with experiment and simpler theories. 

Theoretical models of different complexity are available for Raman CID. They can be used to calculate the Raman optical activity with quantum chemical programs. Though some good results have been reported using semiempirical calculations, the most reliable are the results obtained with ah initio methods, pioneered by Polavarapu (1990). 

D. Goutier, R, Macaulay and A, 3. Duke PHANTOM, Ab initio Quantum Chemical Programs for CDC 6000 and 7000 Series Computers. QCPE 241, Indiana University, Bloomington. 

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