AMSOL

One very popular technique is an adaptation of the Born model for orbital-based calculations by Cramer and Truhlar, et. al. Their solvation methods (denoted SMI, SM2, and so on) are designed for use with the semiempirical and ah initio methods. Some of the most recent of these methods have a few parameters that can be adjusted by the user in order to customize the method for a specific solvent. Such methods are designed to predict ACsoiv and the geometry in solution. They have been included in a number of popular software packages including the AMSOL program, which is a derivative of AMPAC created by Cramer and Truhlar. 

The SM1-SM3 methods model solvation in water with various degrees of sophistication. The SM4 method models solvation in alkane solvents. The SM5 method is generalized to model any solvent. The SM5.42R method is designed to work with HF, DFT or hybrid HF/DFT calculations, as well as with AMI or PM3. SM5.42R is implemented using a SCRF algorithm as described below. A description of the differences between these methods can be found in the manual accompanying the AMSOL program and in the reviews listed at the end of this chapter. Available Hamiltonians and solvents are summarized in Table 24.1. 

Three popular semiempirical programs, AMP AC, AMSOL, and MOPAC, are actually derivations of the same original code. AMP AC 1.0 and MOPAC 3.0 were both created from Version 2.0 of MOPAC. AMSOL was derived from... 

The most common approach to solvation studies using an implicit solvent is to add a self-consistent reaction field (SCRF) term to an ab initio (or semi-empirical) calculation. One of the problems with SCRF methods is the number of different possible approaches. Orozco and Luque28 and Colominas et al27 found that 6-31G ab initio calculations with the polarizable continuum model (PCM) method of Miertius, Scrocco, and Tomasi (referred to in these papers as the MST method)45 gave results in reasonable agreement with the MD-FEP results, but the AM1-AMSOL method differed by a number of kJ/mol, and sometimes gave qualitatively wrong results. 

ArH = dimethoxybenzenes, amsol, xylenes, 50-99% fluorene, diphenyl ether, etc R = alkyl, aryl... 

The AMSOL model and the related SMx methods [22] are based on semi-empirical quantum chemical calculations. Normally these models use a GB approximation for the dielectric contribution of AG of solvation, but COSMO has also been used for one parameterization. In order to overcome the large electrostatic errors of bare semi-empirical methods, charge models have been developed, which improve the electrostatic... 

All calculations were performed with the AMSOL 6.5.3 package [42] on the Cray J916 in Poznan Supercomputing and Networking Center. 

Effects of solvation on zwitterion formation between methylamine and fom-aldehydewere studied by various solvation methods. The SM2/AM1 model predicted the expected zwitterionc minimum while SM3/PM3 failed to do so [127]. Calculations were performed with the use of AMSOL to account for solvation effects in the study of molecular properties and pharmacokinetic behavior of ce-tirizine, a zwitterionic third-generation antihistaminic. Results indicated that the folded conformation remains of low energy not only in vacuo but also in water solution [128]. 

It is worth mentioning that anew quantum mechanical and molecular mechanical method was developed by Kaminski and Jorgensen, based on the CM1A charges computed by AMSOL [61]. Results obtained with this method demonstrated the proposed approach provides results competitive with other methods for estimating solvent effects ontautomerization [137]. 

TABLE 3. AMSOL calculations for the dinmidc of (R Ji)-tartaric acid in water solution... 

In water solution, as shown by AMSOL calculations, the conformational preferences of the studied amides show up. Gibbs free energies ofhydratation calculated with the SM5.4 model and PM3 hamiltonian for structures optimized at the RHF/6-3 1G level indicated that those most favored by hydratation are the T- and G-conformers for the diamide and A, A,A ,A -tetramethyldiamide of (R, R (-tartaric acid, respectively. 

Solvation effects, as calculated with the use of the AMSOL method, affect the confomational preferences of (i ,i )-tartaric acid amides, so that their lowest-energy structures are different from the G+aa conformers favored in isolated molecules. Hydratation favores the Tact conformation for the primary amides and the G-p+p+ conformation for the tertiary amides of (R,R (-tartaric acid. 

The models are all available in the semiempirical package AMSOL.339 They have also been implemented in commercial software packages.299.34o... 

In addition to ab initio methods, developments for semiempirical methods are being investigated on parallel machines. Notable in this regard are MOPAC > 20 (at the San Diego Supercomputer Center i EPSRC Dares-bury Laboratory ) and AMSOL.122 expected, many of the problems encountered with ab initio codes are also found with these programs. 

C. J. Cramer, private communication, 1993. AMSOL is based on AMPAC, with the inclusion of solvation effects. A fully parallel version of this code is under development by Cray Research Inc. See also, C. J. Cramer and D. G. Truhlar, Chapter 1, this volume. Continuum Solvation Models Classical and Quantum Mechanical Methods. 

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