Harmonic force fields, semiempirical

To estimate the parameters of the harmonic force fields that may appear from the SLG-based semiempirical treatment, we consider the symmetric correlated single bond, where the energy can be obtained without any reference to its environment. In our case the derivative of the bond energy with respect to a geometry parameter q has the form ... 

Harmonic force fields are commonly calculated from numerical finite differences of gradients in semiempirical programs, and from analytic... 

The temperature dependence of QI rate constant, computed in Ref. [45], is shown in Figure 3.7. QI CCSD + B98 (HA) curve denotes HA result based on V2 (i.e., B98), and QI CCSD + B98 (HA) + EVB (PI anharm.) curve shows the full QI result, that is, including the anharmonicity PI correction computed with V3 (i.e., an EVB based on GAFF force field [86]). As GAFF force field considers only harmonic stretches, the only anharmonicities included are due to nonbonded interactions. Because of this, the final and presumably the most accurate curve is QI CCSD - -B98(HA) + EVB AMI (PI anharm.) that stiU uses EVB potential near the transition state, but computes the reactant PI anharmonicity corrections with the semiempirical AM 1 potential. It may seem more consistent to use AM 1 potential also in the constrained PI simulations however, this was computationally prohibitive. For simplicity, in the following discussion, CCSD + B98 (HA) is omitted as this part is common to all theoretical methods considered. 

The main drawback of the GVPT2 and DVPT2 treatments is their direct dependence on the semiempirical thresholds, which may lead to inconsistencies for example, when anharmonic frequencies computed using different force fields are compared. It can occur that the harmonic frequency differences and the criteria defined in Eqs. 10.29-10.32 are close to the threshold values, so that different resonant dyads are identified with different methods. Moreover, when reaction paths are considered, it can occur that for two structures on the path, different resonant dyads are found, leading to noncontinuous thermodynamic functions. For these reasons, it is desirable to derive more robust resonance-free models within the VPT2 theory. 

---