Virtual bond variable

Figure 3. Two alternate methods of describing an a-D-glucose residue using the variable virtual bond method. The bond length, bond angle, and conformation angle for atom 05 are shown as ts, 6s, s-...

In practice, each CSF is a Slater determinant of molecular orbitals, which are divided into three types inactive (doubly occupied), virtual (unoccupied), and active (variable occupancy). The active orbitals are used to build up the various CSFs, and so introduce flexibility into the wave function by including configurations that can describe different situations. Approximate electronic-state wave functions are then provided by the eigenfunctions of the electronic Flamiltonian in the CSF basis. This contrasts to standard FIF theory in which only a single determinant is used, without active orbitals. The use of CSFs, gives the MCSCF wave function a structure that can be interpreted using chemical pictures of electronic configurations [229]. An interpretation in terms of valence bond sti uctures has also been developed, which is very useful for description of a chemical process (see the appendix in [230] and references cited therein). 

This shielding effect and variable conformation of bonded ligands makes the estimate of the amount of accessible residual silanols virtually impossible. 

We are thus led to assume that V and the fiuctuation-dissipation process driving the virtual body depends on the variable 17 so as to simulate the efiects of the H-bond dynamics. For example, a strong interaction potential V accompanied by friction and stochastic torques (forces) of weak intensity simulates the solidlike properties of the environment when the tt ed molecule is characterized by four hydrogen bonds. In this case a reasonable approximation is to simulate such an environment by ice Ih. In the opposite limit, a weak potential V with strong friction and stochastic torques (forces) simulates the difiusional properties of the unbounded molecule (liquid water at high temperature or in very dilute nonpolar solution). 

It is found, in particular, that when using the well-known model of the itinerant oscillator, one cannot give up the assumption that the interaction between real and virtual variables is linear without also making this interaction fluctuate randomly in time. This establishes the link with Chapter VII. This fluctuating process can be used to model the influence of hydrogen bond dynamics, the long-time effects of which are then carefully explored and... 

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