About Gaussian 03W :
Gaussian 03 is the latest in the Gaussian series of electronic structure programs. Gaussian 03 is used by chemists, chemical engineers, biochemists, physicists and others for research in established and emerging areas of chemical interest.
Starting from the basic laws of quantum mechanics, Gaussian predicts the energies, molecular structures, and vibrational frequencies of molecular systems, along with numerous molecular properties derived from these basic computation types. It can be used to study molecules and reactions under a wide range of conditions, including both stable species and compounds which are difficult or impossible to observe experimentally such as short-lived intermediates and transition structures. This article introduces several of its new and enhanced features.
Traditionally, proteins and other large biological molecules have been out of the reach of electronic structure methods. However, Gaussian 03's ONIOM method overcomes these limitations. ONIOM first appeared in Gaussian 98, and several significant innovations in Gaussian 03 make it applicable to much larger molecules.
This computational technique models large molecules by defining two or three layers within the structure that are treated at different levels of accuracy. Calibration studies have demonstrated that the resulting predictions are essentially equivalent to those that would be produced by the high accuracy method.
The ONIOM facility in Gaussian 03 provides substantial performance gains for geometry optimizations via a quadratic coupled algorithm and the use of micro-iterations. In addition, the program's option to include electronic embedding within ONIOM calculations enables both the steric and electrostatic properties of the entire molecule to be taken into account when modeling processes in the high accuracy layer (e.g., an enzyme's active site). These techniques yield molecular structures and properties results that are in very good agreement with experiment.
For example, researchers are currently studying excited states of bacteriorhodopsin (illustrated below) using an ONIOM(MO:MM) model, as a first step in understanding the means by which this species generates energy within a cell. In this two-layer approach, the active site is treated using an electronic structure method while the rest of the system is modeled with molecular mechanics. Electronic embedding, which includes the electrostatics of the protein environment within the QM calculation of the active site, is essential to accurate predictions of the molecule's UV-Visible spectrum.
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