The IEPA Method

Progress in the calculation of complete potential energy surfaces can best be followed by considering in detail the work that has been done on one of the simplest of exchange reactions, namely H + + H. 

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Finally, Keil and Ahlrichs,249 reported PNO-CI and CEPA calculations on several Sn2 reactions, including that giving CH5-. The barrier height computed was 236 kJ mol-1, and the inclusion of correlation decreased the barrier by ca. 30 kJ mol-1. The larger drop found in ref. 245 was probably an artefact of the IEPA method. The errors in these very extensive calculations were believed to be only 10—30 kJ mol-1, which is indicative of the accuracy now attainable. 

At present the best wavefunction for H2O using conventional methods is that obtained by Meyer, and reported in 1971.484 The results of this extensive calculation have been thoroughly discussed by Schaefer 1 here we just note that 84% of the correlation energy was obtained Lishka,193 however, in a very recent paper, using the IEPA method, has almost obtained as low an energy, and has discussed theoretically the proton affinity of OH-. The best results for the stretching force constant are within 5 % of the experimental value. However, R(0—H) is now too long by about the same amount as it is too short in SCF calculations. 

Lishka and Dyczmons have gone beyond the SCF calculations, with larger and more flexible basis sets, and have included correlation by the IEPA method.586 The equilibrium geometry is pyramidal in both sets of calculations, the correlation results... 

The Dunham method of analysis of PE curves is well known, but Simons and Parr197 have presented an alternative method. Calculations on HF and CO show the method [which involves the expansion parameter (R — Re)/R instead of (R—Re) I Re] to be superior. The IEPA calculations by Lishka193 also included applications to HF. His total energy of —100.4005 hartree was only 0.03 hartree from Bender and Davidson s large Cl result.142... 

A. He/H2 and Ha.—It is convenient to consider first these species. Tsapline and Kutzelnigg375 have applied the IEPA-PNO method, previously described, to the ground state of the He/Ha system. The van der Waals minimum was computed, using a gaussian lobe basis set with carefully optimized exponents. The collinear arrangement with a depth of 21 K was found for the van der Waals minimum, with a saddle point of 14 K for the Czv geometry. The computed surface was compared with experiment and with the R 6 term. The anisotropy of the potential is larger than that predicted asymptotically. 

However, the most extensive calculations on this problem are those using the IEPA, CEP A, and PNO-CI approaches,625 i.e. explicitly including correlation. The results of all three methods were in good agreement, and the effect of electron correlation is rather small for this system. There have been several other papers on hydrogen bonding, and Van Niessen626 has also studied the HF dimer. 

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