Quantum chemistry software

Damle P, Ghosh AW, Datta S (2002) First-principles analysis of molecular conduction using quantum chemistry software. Chem Phys 281 (2—3) 171—187... 

It is not possible to use normal AO basis sets in relativistic calculations The relativistic contraction of the inner shells makes it necessary to design new basis sets to account for this effect. Specially designed basis sets have therefore been constructed using the DKH Flamiltonian. These basis sets are of the atomic natural orbital (ANO) type and are constructed such that semi-core electrons can also be correlated. They have been given the name ANO-RCC (relativistic with core correlation) and cover all atoms of the Periodic Table.36-38 They have been used in most applications presented in this review. ANO-RCC are all-electron basis sets. Deep core orbitals are described by a minimal basis set and are kept frozen in the wave function calculations. The extra cost compared with using effective core potentials (ECPs) is therefore limited. ECPs, however, have been used in some studies, and more details will be given in connection with the specific application. The ANO-RCC basis sets can be downloaded from the home page of the MOLCAS quantum chemistry software (http //www.teokem.lu.se/molcas). 

As a final example in these lectures we shall go through in some detail an MCSCF study of the C3N radical. The method chosen will be CASSCF and the calculations will be performed using the Lund quantum chemistry software... 

Unfortunately, as we explained in our previous report, much contemporary quantum chemistry software does not satisfy these requirements of transparency, accountability and, therefore, accessibility. 

The Quantum Chemistry Program Exchange QCPEf was an early attempt to formalize the distribution of quantum chemistry software. It was created in 1962 by H. Shull ay Indiana University and offered a small collection of some 33 pieces of software. 

Much contemporary quantum chemistry software does not satisfy these requirements. 

For many years quantum chemistry has been one of the primary areas of application of computers in the scientific research. The Schrodinger equation for any molecular system can be easily written down. In principle, the solution of this equation yield the structure and properties of a molecule, but in practice this can lead to severe computational demands which may, in fact, render calculation for particular properties of particular systems intractable. It is important that quantum chemistry software be efficient. Thorough documentation of code is an essential ingredient of efficient software. 

Quantum chemistry software that exploits the capabilities of modern GPUs has only recently started to emerge. Significant parts of these initial efforts have been devoted to minimize errors caused by the lack of DP support on older GPUs. The advent of next-generation GPUs that support DP arithmetics at a peak performance of only a factor of 2 less than that of SP will make these special approaches obsolete. At the same time, future developments will be greatly facilitated. 

We are looking forward to exciting new developments of quantum chemistry software for GPUs accompanied by ground-breaking applications in the near future. 

From eq. (15) it is clear that also the derivatives with respect to coordinates of nuclei of above expressions are required. The technique of evaluation of the hybrid coulombic and exchange integrals (18) and their derivatives can be found in the literature16,17 18. Matrix of the normal modes dRfin /5density matrix Pfiv are calculated by a standard quantum chemistry software for the evaluation of vibrational frequencies of molecules. 

The exact energy functional (and the exchange correlation functional) are indeed functionals of the total density, even for open-shell systems [47]. However, for the construction of approximate functionals of closed as well as open-shell systems, it has been advantageous to consider functionals with more flexibility, where the a- and j8-densities can be varied separately, i.e. E[p, p ]. The variational search for a minimum of tire E[p, p ] functional can be carried out by unrestricted and spin-restricted approaches. The two methods differ only by the conditions of constraint imposed in minimization and lead to different sets of Kohn-Sham equations for the spin orbitals. The unrestricted Kohn-Sham approach is the one most commonly used and is implemented in various standard quantum chemistry software packages. However, this method has a major disadvantage, namely a spin contamination problem, and in recent years the alternative spin-restricted Kohn-Sham approach has become a popular contester [48-50]. 

Abstract Data Types In The Construction Of Knowledge-Based Quantum Chemistry Software... 

A method to deal with this problem was solved by P.-A. Malmqvist 20 years ago [64,65]. The method has become known as the CASSCF State Interaction (CASSI) method and is effective also for long CAS-CI expansions. It was recently extended to deal also with the integrals of the spin-orbit Hamiltonian [66]. The whole approach has been implemented in the latest version of the MOLCAS quantum chemistry software [67]. 

The excitation energies are obtained as the MS-CASPT2 energy difference between the excited state and the ground state computed with the same active space. The transition moments have been computed from the CASSCF wave functions. This is usually a reasonably accurate procedure. If the MS-CASPT2 treatment shows appreciable mixing between different CASSCF wave functions, we use instead these perturbation mixed functions (PM-CAS) to compute the transition properties. As we shall see, such a procedure becomes necessary for the Bi states. All calculations were performed using the MOLCAS quantum chemistry software [67]. 

Molecular orbital theory differs from valence bond theory in that it does not require the electrons involved in a bond to be localized between two of the atoms in a molecule. Instead, the electron occupies a molecular orbital, which may be spread out over the entire molecule. As in the valence bond approach, the molecular orbital is formed by adding up contributions from the atomic orbitals on the atoms that make up the molecule. This approach, which does not explicitly model bonds as existing between two atoms, is somewhat less appealing to the intuition than the valence bond approach. However, molecular orbital calculations typically yield better predictions of molecular structure and properties than valence bond methods. Accordingly, most commercially available quantum chemistry software packages rely on molecular orbital methods to perform calculations. 

Thus, the more and more widespread use of quantum chemistry software has not been matched by an increase in the understanding of the theoretical models and methods of implementation of these models which are implicit in that software. 

The number of references has been purposely kept small in the hope that they will be actually consulted. However, there is an enormous amount of material available bearing on the theory of molecular electronic structure and applications of various models to specific cases, There is, on the other hand, a very much smaller literature on the actual implementation of the methods. Generally speaking, literature on the use of modern software tools in the generation and maintenance of quantum chemistry software is rather thin on the ground. 

Here are a few facts in random order about existing quantum chemistry software ... 

Through a solid assessment of parallel computing hardware issues, parallel programming practices, and implementation of key methods, this invaluable book enables readers to develop efficient quantum chemistry software capable of utilizing large-scale parallel computers. 

E. Hiickel introduced a simple quantum mechanical model for the description of the electronic structure of planar unsaturated molecules with the bonding connectivity as input. This model has been widely used. Although today s computing power and quantum chemistry software available for all chemists have made the assumptions of the Hiickel model unnecessarily simplistic, the model is still used to make estimates of molecular energies and has established itself as a useful teaching tool. 

Nowadays, several companies sell quantum-chemistry software for doing molecular quantum-chemistry calculations. These programs are designed to be used by all kinds of chemists, not just quantum chemists. 

A new set of calculations was performed for the present discussion. This were done on a Linux-equipped laptop. The MOLCAS quantum-chemistry software was used. The basis set was of the... 

Roos, B. O. These calculations were performed with the CASSCF module of the MOLCAS quantum chemistry software. The active space chosen was the 3d and orbital of each Cr atom (12) with 12 active electrons.. 

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