Atom-atom potentials representing separate contributions to the nonempirical interaction energy have been derived in the SCF decomposition scheme corrected for basis set superposition error by the counterpoise method. The nontransferable long-range electrostatic multipole and classical induction terms have been evaluated directly from cumulative atomic multipole expansions, whereas the short-range exchange, charge-transfer, and electrostatic penetration contributions have been represented by simplified potentials of the form ( p + 6R-l) exp(-6R) fitted to the corresponding ab initio results for 336 dimer configurations formed by HF, HzO, NH3, CHI, CO, and COz. The dominant anisotropic character of electrostatic multipole atom-atom potentials and much more isotropic nature of the potentials representing short-range terms is illustrated in the Appendix for head-on interactions in CO . . OC and HF . . FH dimers.

The SCF potential energy surface for H+H2 is reported for atom–molecule distances R in the range 2.5–6.0a0 and diatomic bond lengths r between 1.2a0 and 1.6a0. The leading harmonic components V0(R, r) and V2(R, r) are obtained. The SCF energies are estimated to be within 2 meV of the Hartree–Fock limit, while the error in V0 is estimated to range from 0 to 7 meV, due mainly to the neglect of higher harmonic components.

To demonstrate the feasibility of quantum chemical and other theoretical techniques in the understanding and correlation and then in the prediction of toxicity of a specific class of compounds, the organophosphorus acetylcholinesterase (AChE) inhibitors were chosen since these have relatively clear-cut one-dose acute lethal toxicity, both LD50, and ED50 for the triggering event inhibition of AChE, to serve as well-defined end points. It was possible to identify particular quantum chemical requisites which are related to and predictive of the toxic effects of these molecules. A b initio calculations were carried out on a number of organophosphorus compounds using our own programs which incorporate a number of desirable options for ab initio calculations on large molecules, including ab initio effective core model potentials (MODPOT), which permit calculations of valence electrons only explicitly, yet accurately, and a charge-conserving integral prescreening evaluation (which we named VRDDO-variable retention of diatomic differential overlap) especially effective for spatially extended molecules. Ab initio calculations were carried out for the two optical isomers of GB:H,C-P-0-CH FThese isomers differ in their toxicities. From the wavefunctions of the optical isomers of GB (calculated relative to internal body-fixed coordinates) the electrostatic molecular potential contour maps (relative to the same absolute space-fixed reference coordinates) were generated in three dimensions around each molecule. These maps indicate vividly the differing stereoelectronic requisites for the different optical isomers and suggest the basis for the difference in their in uiuo toxicities. These maps form three-dimensional stereoelectronic holograms and can even be used for reverse image holography to ascertain if new even hypothetical molecules fulfill the requisites. This is related

The first step in the inhibition of acetylcholinesterase (AChE) by organophosphorus anticholinesterases is formation of the organophosphorous molecule-AChE complex. In the first paper of this series it was shown that the first step can be modeled well by the use of three-dimensional electrostatic molecular potential contour maps around the P compound. The second phosphorylation step of the mechanism by which organophosphorous anticholinesterases of the general formula 0 II R,-P-OR,' I inhibit AChE is related to the acidity of X and to the bond strength of the P-X bond. For a series of compounds, aryl N-methyl methyl phosphoramidates, in which the P-X bond is constant, this paper shows that the quantum chemically calculated P-0 total overlap populations (TOPS) correlate the experimental log 150 for inhibition of AChE over four orders of magnitude with correlation coefficients of 0.96 [r2: TOP 0.9581; (TOP)'12 0.96141.