Supermolecule calculation

Much effort has been devoted to other electrostatic representations of molecular interactions [89] using ab initio calculations based on the understanding that this component is the largest portion of the interaction energy [90]. A major application of the detailed analysis of intermolecular interactions provided by ab initio formulations has been an approximate expansion in terms of analytical functions that allow practical calculation for many different intermolecular distances [97], Alarge scale simulation of potential functions for solvated amino acids has been derived from supermolecule calculations based on one interacting water molecule [92],... 

Notes Results under this heading are from supermolecule calculations. Change [[ab initio force constant for molecule with hydration/afr imr/o force constant molecule with no hydration (from column 1)]- 1.0 x 100. 

With the success of these calculations for isolated molecules, we began a systematic series of supermolecule calculations. As discussed previously, these are ab initio molecular orbital calculations over a cluster of nuclear centers representing two or more molecules. Self-consistent field calculations include all the electrostatic, penetration, exchange, and induction portions of the intermolecular interaction energy, but do not treat the dispersion effects which can be treated by the post Hartree-Fock techniques for electron correlation [91]. The major problems of basis set superposition errors (BSSE) [82] are primarily associated with the calculation of the energy. 

This technique should be rather taken as a tool giving us information about the positions that should be selected for a complete SCF supermolecule calculation. 

Technical aspects of supermolecule calculation need not be noted specifically since they are essentially the stitne as those mentioned in the previous section. Hints for the basis set selection given in Table 5,16 may be supplemented with a note referring to proton potential curves. Small basis sets give potential curves with a single minimum... 

Thus perturbation theory calculations currently suffer from the same disadvantage of supermolecule calculations for defining the intermolecular potential between organic molecules at short range. They are so expensive when used to obtain results of reasonable accuracy that are converged with respect to basis set, even for small molecules, that it is impossible to calculate the energies at a sufficient number of points to define the potential energy surface. Calcula-... 

The SCRF model reproduces very well the shifts observed in aptotic solvents. In order to simulate an aqueous solution, calculations on supermolecules containing the chromophore and specifically bound water molecules, all embedded in the dielectric continuum, are required even for merely qualitative agreement. Supermolecule calculations with several water molecules treated as a gas-phase cluster often give qualitatively incorrect results (Karelson and Zerner, 1992). 

We are examining the question of various rigorous perturbation expressions (91) for intermolecular interactions and are currently doing the analysis and writing a program to calculate these (92). We shall test the validity of this latter procedure by comparison with accurate ab-initio supermolecule calculations for smaller test systems. 

When estimating the polyhydration energy of the tautomer, one can also perform supermolecule calculations. In this model... 

As previously stated, truncated Cl s are neither size extensive nor size consistent. A simple (and often used) example makes this clear. Consider two noninteracting hydrogen molecules. If the CISD method is used, then the energy of the two molecules at large separation will not be the same as the sum of their energies when calculated separately. For this to be the case, one would have to include quadruple excitations in the supermolecule calculation, since local double excitations could happen simultaneously on A and B. 

SAPT avoids the subtraction of large energy values that is necessarily part of a supermolecule ab initio calculation. A supermolecule calculation obtains the interaction energy of monomers A and B, AVab, as Tab - Va - Vb. whereas SAPT finds AVab directly. The interaction evaluated in SAPT is defined so as to be free of BSSE however, the other requirements on basis-set quality and for correlation effects still hold. SAPT has yielded highly accurate interaction data, first for rare gas atoms interacting with small molecules [72 74] and more recently with molecule molecule clusters such as the CO2 dimer [75]. Further examples are the very accurate results achieved for Ne HCN [76] and a parr potential for water [77]. Another example study of perturbative treatment of the interaction potential has been a study of rare gas HCN clusters [78] which included vibrational analysis. 

The basic strategy of the method was proposed by Wormer and van der Avoird who first applied it to the case of the He-He interactionThey showed that very simple VB wavefunctions provided the self-same interaction energies as fairly sophisticated MO-CI (supermolecule) calculations. Of... 

---