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603. NOEL: Number of Overlapping Electrons - A Molecular
Similarity Index
by J. Cioslowski, Department of Chemistry and Supercomputer Computations Research Institute, Florida State University, Tallahassee, Florida 32306 To calculate the degree of similarity between two molecular systems, the molecular similarity index, called the number of overlapping electrons (NOEL), is defined as overlap between the first-order reduced- density matrices of the systems in question. Although the methodology is general, the current version of NOEL is limited to the density matrices derived from the RHF or ROHF wavefunctions calculated by the GAUSSIAN 88 suite of programs.NOEL reads the necessary information from two CHECKPOINT files generated by GAUSSIAN 88 and inquires about the starting orientation of the molecules. The similarity index is then maximized by optimizing the translations and rotations describing the mutual orientation of molecules. Depending on the initial orientation, NOEL arrives at different local (or global) maximum.Chemical significance of the maxima is then assessed by the user. This assessment is aided by the fact that, for the HF wavefunctions, the value of NOEL is equal to the average number of electrons present in the fragment shared by the molecules. The cost of calculations is proportional to the third power of the sizes of the molecules under study. The maximum similarity orbitals are also calculated if requested. Reference: J. Cioslowski and E. D. Fleischmann, J. Am. Chem. Soc., 113, 64 (1991). NOTE:For matters of computational convenience, QCPE will distribute this system in VAX COPY format only. We will distribute both the VAX and the CRAY Y-MP versions together. Please note that you must have GAUSSIAN 88 running if you intend to use the system as we are distributing it. GAUSSIAN 88 is not available through QCPE. Lines of Code: 2040 FORTRAN 77 Libraries Used: UTIL of GAUSSIAN 88 Limitations: Maximum of 255 basis functions. Other limitations imposed by GAUSSIAN 88. |