Calculated molecular ground-state

Clever use of this approach and choice of exchange correlation potential has produced methods for calculating molecular ground state properties and energetics in reasonable agreement with experimental numbers for an impressive array of systems. The fact that these methods are computationally inexpensive make them applicable to larger systems than can routinely be studied with proper quantum mechanical approaches. This makes them useful simulation tools. 

Table III. Calculated Molecular Ground-State Properties...

In order to consider the inversion of Qx(0,0) and Qy(0,0) electronic transition intensities in NH-tautomers of non-symmetrical free-base porphyrins we calculated the ground-state orbital energies of the investigated molecules by a CNDO/2 method using the symmetrized crystal geometry of porphyrin molecule (37,38). On the basis of the above experimental results we must introduce a motionless system of molecular X and Y axes, identically fixed in both tautomers. Then using theoretical MO calculations and the analysis... 

In the final exploration of the quantum chemistry unit students use a computational chemistry package (eg. Spartan, Gaussian, CaChe, etc.) to calculate the ground state energies, molecular orbitals, and in some cases the excited state energies, of two proton transfer tautomers. Calculations are performed at several different levels of theory, and use both semi-empirical and ab initio methods. Several different basis sets are compared in the ab initio calculations. The students use the results of these calculations to estimate the likelihood of excited state proton transfer. The calculations require CPU time ranging from a couple of minutes to a couple of hours on the PCs available to the students in the laboratory. 

Fig. 3. MOs for the molecular ground state of Ni(CO)4 from different types of Xq, calculation. (From Ref. 194.)...

The Hartree—Fock calculations (97,179, 180) on Ni(CO)4 agree with each other quite well in terms of the predicted sequence of MOs in the molecular ground state (Table I). Both calculations support the postulate of some it back bonding. Calculations of the first two ionic states of Ni(CO)4 have also been done (177) (Table I), revealing that substantial relaxation energies (>5 eV) are associated with the predominantly metal 912 and 2e MOs. 

Deviations between calculated and experimental values are generally smaller than 4 ppm [306], Differences may be attributed partly to varying molecular ground states and partly to varying excited states of a, / -unsaturated acids [306]. 

The construction of exchange correlation potentials and energies becomes a task for which not much guidance can be obtained from fundamental theory. The form of dependence on the electron density is generally not known and can only to a limited extent be obtained from theoretical considerations. The best one can do is to assume some functional dependence on the density with parameters to satisfy some consistency criteria and to fit calculated results to some model systems for which applications of proper quantum mechanical theory can be used as comparisons. At best this results in some form of ad-hoc semi-empirical method, which may be used with success for simulations of molecular ground state properties, but is certainly not universal. 

We have performed a series of semiempirical quantum-mechanical calculations of the molecular hyperpolarzabilities using two different schemes the finite-field (FF), and the sum-over-state (SOS) methods. Under the FF method, the molecular ground state dipole moment fJ.g is calculated in the presence of a static electric field E. The tensor components of the molecular polarizability a and hyperpolarizability / are subsequently calculated by taking the appropriate first and second (finite-difference) derivatives of the ground state dipole moment with respect to the static field and using... 

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