Geometry and Force Field

The experimental geometries and force fields may be used if available, although this is rarely the case for the force fields. In some cases, corrected ab initio geometries, where systematic deviations are known to occur, are used. 

If possible, the MP2/6-31G level, or density functional methods, should be used to obtain geometries and force fields. However, calculations at the SCF level with the basis set denoted 6-31G -3) have been found to yield geome- 

Ideally, for magnetic properties, gauge-origin independent atomic orbitals (GIAO) should be used as discussed earlier. These orbitals reduce the sensitivity of calculated properties to the size of the basis set and therefore permit applications to larger molecules. The use of GIAO for VCD calculations has recently been reported.  

These data are written to the read-write file from which, in the final step, they are fed into the VCT90 program in which the actual computation of VCD intensities based on the VCT formalism is accomplished. 

The MFP approach was iSrst implemented in a modified Gaussian 80 program, and a finite di rence method was used to compute the wavefunction derivatives. The MFP formalism has been implemented in the CADPAC ab initio program s and is fully analytical. An MFP implementation at the SCF level has also been carried out by Morokuma and Sugeta. An implementation of MFP using CASSCF wavefunctions and gauge-invariant orbitals has been accomplished by Bak et al.  

---

Fluorine ions form the first coordination sphere around the tantalum ion, which is the central atom of the complex. Potassium ions form the second coordination sphere, which significantly affects the geometry and force field of the first coordination sphere. The melting of K2TaF7 leads to the dissociation of the compound into ions, as follows ... 

It is especially important to investigate the molecular structure of coordination compounds in the vapor phase because the relatively weak coordination interactions may be considerably influenced by intermolecular interactions in solutions and especially in crystals. It has been shown that the geometrical variations can be correlated with other properties of the molecular complexes ). In particular the structural changes in the F3B N(CH3)3 and CI3B N(CH3)3 molecules ) relative to the respective monomeric species unambiguously indicated boron trichloride to be a stronger acceptor than boron trifluoride. Data on the geometry and force field have also been correlated ). 

Acetic anhydride in the gas phase, studied by electron diffiaction and infrared spectroscopy, supplemented with ab initio calculations of geometries and force fields ... 

The VCD of 7 was recalculated in 1988 with the MFP method using analytical methods as implemented in CADPAC rather than finite difference methods. The DO gauge and substantially large basis sets were employed. The VCD spectrum was also remeasured and rotatory strengths were obtained. The basis set, geometry, and force field dependence the VCD intensities ot this molecule were investigated in those studies. 

As the first VCD study partially at the MP2 level of theory, Amos et al. studied the VCD of 11 using the MFP method with the force field and APTs at the MP2 level, and AATs at the SCF level.The equilibrium geometries and force fields were obtained with a variety of basis sets, and only the results obtained with a DZP basis set were quoted. The authors concluded that the correlation effects improve both the APTs and frequencies, and that correlation affected frequencies more than intensities in the C-H stretching region. 

M. A. Lowe and J. S. Alper, /. Phys. Chem., 92, 4040 (1988). Ab Initio Calculations of Vibrational Circular Dichroism in Propylene Oxide Geometry and Force Field Dependence. 

So electronic transitions of anisole [53] represents an example of the accuracy achievable when time-independent simulations of vibronic spectra are coupled to good-quality ab initio computations for geometries and force fields in both electronic states. For anisole, methods based on the density functional theory and its time-dependent extension for electronic excited states [B3LYP/6-311 +G(d,p) and TD-B3LYP/6-311 +G(d,p)] have been applied to perform geometry optimizations and harmonic frequency calculations, while the energy of the electronic transition has been refined by EOM-CCSD/CCSD//aug-cc-pVDZ computations. The remarkable... 

In order to approach chemical accuracy for equilibrium geometry and force field predictions, it seems necessary to include at least /-type functions in the basis set for heavy atoms and usually J-type functions for hydrogen. The use of smaller basis sets inevitably leads to less accurate predictions, even with highly correlated wave functions, like CCSD(T), as can be seen in Table 8. Although correlation consistent " and ANO basis sets are rather similar in quality, the cost of the computation of integrals based on the former choice is significantly less. Thus, they have been used exclusively to obtain the results presented in Table 8. 

---