COSMIC force field

Correlation functions, in simulations, 380 Correlation consistent basis sets, 162 COSMIC force field, 40 Coulomb correlation, 99... 

Examples are the Tripos force field (22), the COSMIC force field (23), and that of White and Bovill (24), which uses only two atom types, those at the end of the bond to parameterize the torsional potential rather than the four types of the atoms used to define the torsional angle. One has only to consider the number of combinations of 20 atom subtypes taken four at time (160,000) versus two at a time (400) to understand the explosion of parameters that occurs with increased atom sub-types. The simplifying assumption in parameterization of the torsional potential reduces to some extent the quality of the results (25), but allows the use of the simplified force fields (22) in many situations where other force fields would lack appropriate parameters. The situation can become complicated, however. For example, the amide bond is normally represented by one set of parameters, whether the configuration is cis or trans. Experiments data are quite compelling that the electronic state is different between the two configurations, and different parameter sets should be used for accurate results (Fig. 3.1). Only AM-BER/OPLS currently distinguishes between these two conformational states (26). Certainly, the limited parameterization of simplified force fields would not allow accurate prediction of spectra that is more reflective of the dynamic behavior of the molecule. 

The TRIPOS force fields in the SYBYL and Alchemy programs and the Chem-X, CHARMm, and COSMIC force fields all employ a simple harmonic potential (Eq. [1]) for bond stretching/compression. The CVFF, DREIDING, and UFF force fields support a Morse potential as well as a harmonic potential. The harmonic function is the default in DREIDING and UFF. 

Like MM2, MM3 uses a potential energy function with an exponential function for the repulsive part as shown in Eq. [14]. This type of van der Waals term is also used by the COSMIC force field. 

The different force fields developed over the years differ considerably with respect to types and number of cross-terms. The TRIPOS, Chem-X, CHARMm, DREIDING, UFF, and COSMIC force fields do not include any cross-terms at all, and accordingly they are all purely diagonal force fields. MM2, the MM2-based force fields, and MMFF93 include a single cross-term (a stretch-bend function), whereas MM3 includes stretch-bend, bend-bend, and torsion-stretch terms. CVFF and CFF91 additionally include stretch-stretch, torsion-bend-bend, and other cross-terms. 

Mimic (1.0), and ChemSD Plus (3.0) software, in addition to the abilities of their force fields [TRIPOS, COSMIC, MMX, MM2(87), and MM2, respectively] to reproduce experimental conformational energies. This comparison comprised a series of 31 molecules and their corresponding conformational energy data. In this test, MM2(87) and the two MM2-based methods (MMX and MM2 ) were clearly superior to the TRIPOS and COSMIC force fields. The rms deviations for the MM2 methods were 0.32-0.37 kcal/mol, whereas the corresponding values for the TRIPOS and COSMIC force fields were 0.90 and 0.93 kcal/mol, respectively. 

Tel. 703-658-4854, fax 703-658-4887, e-mail syazdi presto.ig.com Structure building, manipulation. Stick, ball-and-stick, space-filling, and dot surface display. Energy calculations with the COSMIC force field. Partial charges from Charge-2. PC-PROT + (sequence analysis), PC-TAMMO-t- (protein-lipid modeling), and MASCA (statistics). URL is http //www.oxmol.co.uk/. Macintosh and PCs (Windows). 

The quintessence hypothesis was proposed by J. P. Ostriker and Steinhardt (2001). The authors use the term quintessence ( fifth substance ) to describe a quantum force field which is gravitationally repulsive. It has a certain similarity to an electrical or magnetic field and could lead to an invisible energy field which accelerates cosmic expansion. 

The scalar field that is the object of their greatest desires and most ardent research is called the Higgs field. It arises as a logical necessity in their grand unification of the forces of nature. However, the cosmic scalar field mentioned above is of a very different nature, for it is infinitely lighter than the Higgs field. 

COSMIC [181] method uses only harmonic potential with respect to displacements in the diagonal force fields. Neither improper torsion nor cross terms are included. The nonbonding interactions are the sum of van der Waals interactions represented by the Morse potential and the charge-based Coulomb energy. 

The TRIPOS, Chem-X, CHARMm, COSMIC, and CVFF force fields all use only the harmonic approximation, whereas in the DREIDING force field, angle bending is described by a harmonic cosine function where 0 and 0o in Eq. [6] are replaced by cos 0 and cos 0q, respectively, to avoid a zero slope as 0 approaches 180°. In the UFF force field, angle bending is described with a small Fourier expansion in 0 (Eq. [7]). This functional form was selected to better describe large-amplitude motions. 

Besides MM2 and MM3, MMFF93 and CFF91 employ a three-term Fourier expansion, whereas the TRIPOS, Chem-X, CHARMm, COSMIC, DREIDING, and CVFF force fields all use a simple single-term, torsional potential energy... 

It was more than 50 years ago that Yukawa predicted the existence of a new particle, a meson, that should mediate nuclear forces. Before such particle was found in cosmic rays, however, another new particle was discovered. It was quite unexpected and Yukawa him lf at first mistook it for his predicted particle. This new particle was initially called a p meson, but is now known as a muon. Muons have peculiar but useful properties as is described in the following sections and, thanks to the recent development of high energy accelerators, can be obtained in copious amounts. Thus, they have become an important tool in many fields of physical chemistry and materials science. 

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