LEWIS — Generate initial guess based on Lewis dot structure analysis.




The primary purpose of this keyword is to generate an initial set of molecular orbitals and densities as input for the SCF portion of the calculation. (By default, the initial densities are generated as a simple matrix with only elements along the diagonal.) Using LEWIS instead creates an initial guess based on an analysis of the structure's Lewis dot structure. (If the procedure fails, then the diagonal density guess will be used and the calcuation will continue without the LEWIS guess.) The LEWIS guess is intended to provide a better starting guess to reduce the number of SCF cycles needed for convergence (but this is not always the case). Generally, the LEWIS guess is only used for the initial geometry unless the keyword NEWDEN is present.

As part of its algorithm, LEWIS will also generate information about the molecule's electronic configuration (Lewis dot structure) that may be of interest to the user. This is not printed by default and can be found only if PRTLWS=n (or PRINT=n) is used with n at least 1. The analysis (depending on the print level) will include a bond type table, a table showing how the electrons of each atom are distributed into orbitals, and a listing of molecular orbital information. If the print level is high enough, the initial set of molecular orbitals and densities are also printed. If the LEWIS analysis fails or gives unexpected results, this output may help diagnose the problem.

The Lewis dot structure information is then used to construct a set of highly localized orbitals. Each orbital will only extend to one atom (lone pairs, empty orbitals), two atoms (sigma and pi bonds), or three atoms (3 center-2 electron bonds). Highly delocalized aromatic systems are reduced to a set of single and double bonds, which are localized. A complete set of orthonormal orbitals are produced, including both occupied and virtual orbitals. For UHF calculations with equal numbers of alpha and beta electrons, the HOMO and LUMO orbitals are mixed to break the RHF symmetry. The density matrix is constructed from these localized orbitals.

LEWIS uses a heuristic approach and so is not gauranteed to succeed in all cases. It is nevertheless fairly robust and can handle a wide range of molecues and geometries. The most likely cause of failure is highly distorted geometries, where it is difficult for AMPAC to accurately identify the molecule's bonding patterns. As such it is not recommended with simulated annealing (see Chapter 13, Simulated Annealing) and other methods that may generate highly unusual configurations. In general, it is able to handle, highly charged systems, high spin systems, d-orbitals, 3 center-2 electron bonds, highly conjutated aromatic systems, and noble gases.





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