Crude BO approximation

An alternative approximation scheme, also proposed by Bom and Oppenheimer [5-7], employed the straightforward perturbation method. To tell the difference between these two different BO approximation, we call the latter the crude BOA (CBOA). A main purpose of this chapter is to study the original BO approximation, which is often referred to as the crude BO approximation and to develop this approximation into a practical method for computing potential energy suifaces of molecules. 

Nevertheless, the examination of the applicability of the crude BO approximation can start now because we have worked out basic methods to compute the matrix elements. With the advances in the capacity of computers, the test of these methods can be done in lower and lower cost. In this work, we have obtained the formulas and shown their applications for the simple cases, but workers interested in using these matrix elements in their work would find that it is not difficult to extend our results to higher order derivatives of Coulomb interaction, or the cases of more-than-two-atom molecules. 

Table 8.6 traces the evolution of the approximating CA rules as e increases from 0 to 1 for 2 < s < 3. We see that as e increases, only 7 of the 32 possible rules are actually visited. Nonetheless, even at this crude first order approximation, a remnant of the CML s transition to spatiotemporal intermittency remains. In particular, there is a threshold value of e, = 2 — 4/s, that acts as a boundary point below which the approximating CA-rule is simple-periodic (class 1 or 2) and above which it is complex (class 3 or 4). The surprising fact is not that the CML s transition appears to bo approximated by the CA-rule path - after all, allowing for finite-length computer words, the CML itself is essentially just a very-high order CA... 

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