Molecular scattering calculations, selection

Rational Selection of Algorithms for Molecular Scattering Calculations... 

R. G. Gordon, Rational selection of methods for molecular scattering calculations, Disc. Faraday Soc. 55 22 (1973). 

The int rated molecular transform (FT ) is a molecular descriptor calculated from the square ofthe molecular transform, by integrating the squared molecular transform in a selected interval of the scattering parameter s to obtain the area under the curve and finally taking the square root of the area [King, Kassel et al., 1990, 1991]. The square root of the integrated molecular transform, called SQRT index, was also proposed as molecular descriptor [Famini, Kassel et al., 1991]. 

This question can be answered by means of scattering experiments for molecules in fields [1-3] or accurate calculations of the scattering matrices for collisions of molecules in nonperturbative external fields. The purpose of this chapter is to describe the theory for such calculations and present selected results illustrating the effects of external electromagnetic fields on molecular collisions at low temperatures. 

Positron total scattering cross sections have been measured and calculated for a variety of atomic and molecular gases, and in this section we present a selection of results. In the light of the discussion given in section 2.4, particularly concerning small-angle elastic scattering, a critical evaluation... 

The difference between the average of the second law values and those calculated from molecular data is not large, but the second law values are widely scattered due to the small temperature ranges of the measurements. The entropy value in [84PUP/RUS] is selected since it is the result of the best documented molecular calculations,... 

In gas-phase dynamics, the discussion is focused on the TD quantum wave packet treatment for tetraatomic systems. This is further divided into two different but closed related areas molecular photofragmentation or half-collision dynamics and bimolecular reactive collision dynamics. Specific methods and examples for treating the dynamics of direct photodissociation of tetraatomic molecules and of vibrational predissociation of weakly bound dimers are given based on different dynamical characters of these two processes. TD methods such as the direct projection method for direct photodissociation, TD golden rule method and the flux method for predissociation are presented. For bimolecular reactive scattering, the use of nondirect product basis and the computation of the initial state-selected total reaction probabilities by flux calculation are discussed. The descriptions of these methods are supported by concrete numerical examples and results of their applications. 

L. E DiMauro and P. Agostini Infrared Spectroscopy of Size Selected Molecular Clusters, U. Buck Femtosecond Spectroscopy of Molecules and Clusters, T. Baumer and G. Gerber Calculation of Electron Scattering on Hydrogenic Targets, /. Bray and A. T. Stelbovics Relativistic Calculations of Transition Amplitudes in the Helium Isoelectronic Sequence, W R. Johnson, D.R. Plante, and J. Sapirstein... 

A different procedure was used by Lecacheux and Lesec (33) for determining interdetector volume for both a viscometer and a light-scattering detector. In this approach, an excluded monodisperse polymer standard is injected. When the correct interdetector volume is selected, the calculated intrinsic viscosity, or molecular weight, is equal to the expected value and remains constant as a function of elution... 

The calculation of collisional cross sections for phenomena involving atoms and molecules is particularly difficult because many quantum states of the colliding partners are coupled by the interaction forces. Even in cases involving electronically adiabatic phenomena, where one can assume that the electronic states of the system remain the same while the nuclei move, one must yet deal with the coupling of translational, rotational and vibrational degrees of freedom of the nuclei. The interaction forces furthermore depend intricately on the molecular orientations and on the atomic displacements within molecules, and change extensively with the atomic composition of molecules. It is therefore usually impossible to invoke physical considerations to make a preliminary selection of the quantum states that are relevant to the collision. We describe here the computational aspects of an alternative approach, based on the time evolution of operators for scattering, and on their time-correlation functions, which eliminates the need for basis set expansions. 

---