Quantum chemical software

Recent progress in the development of quantum chemical computational methods [1] and quantum chemical software provided a user-friendly tool to chemists, helping them to explain many problems met in a chemical laboratory. The most frequent task is to determine the optimum structure of molecules, relative energies of reaction components, heats of formation, vibrational frequencies, energies of activation and energies of ionization. In all these cases, a standard theoretical approach is to perform first... 

V. A. Zayetz, CLUSTER-ZI Quantum Chemical Software, Institute of Surface Chemistry, National Academy of Sciens of Ukraine, Kiev, 1990. 

Zemer and co-workers had subsequently attempted to use RPA as an alternative to Singles Cl for computing molecular electronic spectra with ZINDO code. " However, historically, these early RPA advances did not develop into standard quantum chemical software. Modern computational packages " usually offer extensive Cl codes but not propagator-based techniques for handling the electronic correlations. However, current studies of propagator techniques - will be gradually incorporated into quantum-chemical software. 

Calculations were performed with the ab initio Hartree-Fock method (4) and in some cases with methods incorporating electron correlation such as MP2 (5) or BLYP (6), Geometry optimizations were initally done with relativistic effective core potential polarized valence double-zeta bases (7) and then refined with standard 6-3IG bases (8). NMR shieldings were evaluated using the GIAO-SCF method (9) and 6-3IG bases. We used the quantum chemical software GAMESS (10) and GAUSSIAN (11). 

This article is the outgrowth of a talk given at the Symposium on Ab Initio Quantum Chemical Software for Supercomputers organized by the Ohio Supercomputer Center, September 7, 1989. 

Even if a quantum chemical software package does not feature an implementation of a given static response property, it is often possible to evaluate the property as an energy derivative by finite difference. Most packages allow the addition of external static fields to the Hamiltonian, making it possible to compute the energy at a specified field strength, (F). From the definition of the partial derivative, we may thus compute the dipole moment and polarizability from finite difference formulas such as... 

The appropriate choice of multi-reference function for quasi-degenerate problems is a significant problem and one which we do not address here. The use of a multireference formalism is required for problems as simple as the dissociation of the ground state of the hydrogen molecule. The choice of multi-reference function is dictated by the physics and chemistry of the systems under study. For more complicated problems the choice of reference requires considerable care. This choice certainly represents a significant barrier to the development of black box quantum chemical software packages for problems demanding the use of a multi-reference formalism. 

Equation (2) was also used to calculate quantum chemical approach. On the basis of previous results [19], calculated electrostatic potentials were computed from ab initio wave functions obtained in the framework of the HF/SCF method using a split-valence basis set (3-21G) and a split-valence basis set plus polarisation functions on atoms other than hydrogen (6-31G ). The GAUSSIAN 90 software package [20] was used. Since ab initio calculations of the molecular wave function for the whole... 

Catalysts were activated before reaction at 450°C in N2 flow for 3 hours. Software Spartan Pro has been used for the quantum chemical semi-empirical (type AM 1) calculation of transition states energies. 

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. 

Recently, quantum chemical computational techniques, such as density functional theory (DFT), have been used to study the electrode interface. Other methods ab initio methods based on Hartree-Fock (HF) theory,65 such as Mollcr-PIcsset perturbation theory,66,67 have also been used. However, DFT is much more computationally efficient than HF methods and sufficiently accurate for many applications. Use of highly accurate configuration interaction (Cl) and coupled cluster (CC) methods is prohibited by their immense computational requirements.68 Advances in computing capabilities and the availability of commercial software packages have resulted in widespread application of DFT to catalysis. 

It is apparent that only a trickle of work has been, and is currently being, done on momentum densities in comparison with the torrent of effort devoted to the position space electron density. Moreover, much of the early work on II( p) has suffered from an undue emphasis on linear molecules. Nevertheless, some useful insights into the electronic structure of molecules have been achieved by taking the electron momentum density viewpoint. The most recent phenomenal developments in computer hardware, quantum chemical methods and software for generating wavefunctions, and visualization software suggest that the time is ripe to mount a sustained effort to understand momentum densities from a chemical perspective. Readers of this chapter are urged to take part in this endeavor. 

Many software packages have been developed for calculation of molecular descriptors. Table 5.5 shows some software employed for molecular modeling, quantum-chemical calculations, molecular dynamics, and QSARs. 

Quantum Chemical Methods and Software for Excited State Energy and Gradient Computations... 

With the advances in experimental solid-state NMR and computational resources (both software and computing power), it is now possible to use both the experimental and computational results (sometimes in a complementary way) to study biologically important macromolecules such as proteins. The quantum-chemical computation (particularly by density functional theory) of NMR parameters in solids has found important application in protein structure determination.30-36 Tesche and Haeberlen37 calculated the proton chemical shift tensor of the methyl groups in dimethyl terephthalate and found the theoretical results were in good agreement with the multiple pulse experiment. 

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