Energy level structure computational methodology

For direct Af-electron variational methods, the computational effort increases so rapidly with increasing N that alternative simplified methods must be used for calculations of the electronic structure of large molecules and solids. Especially for calculations of the electronic energy levels of solids (energy-band structure), the methodology of choice is that of independent-electron models, usually in the framework of density functional theory [189, 321, 90], When restricted to local potentials, as in the local-density approximation (LDA), this is a valid variational theory for any A-electron system. It can readily be applied to heavy atoms by relativistic or semirelativistic modification of the kinetic energy operator in the orbital Kohn-Sham equations [229, 384],... 

CVT approach is particularly attractive due to the limited amount of potential energy and Hessian information that is required to perform the calculations. Direct dynamics with CVT thus offers an efficient and cost-effective methodology. Furthermore, several theoretical reviews60,61 have indicated that CVT plus multidimensional semi-classical tunneling approximations yield accurate rate constants not only for gas-phase reactions but also for chemisorption and diffusion on metals. Computationally, it is expensive if these Hessians are to be calculated at an accurate level of ab initio molecular orbital theory. Several approaches have been proposed to reduce this computational demand. One approach is to estimate rate constants and tunneling contributions by using Interpolated CVT when the available accurate ab initio electronic structure information is very limited.62 Another way is to carry out CVT calculations with multidimensional semi-classical tunneling approximations. 

The title indicates that this paper is about the calculation of vibrational force constants and the geometry optimization of polyatomic molecules however, its primary impact on computational chemistry comes from the methodology for calculating analytic first derivatives with respect to molecular coordinates at the Hartree-Fock (HF) level of theory. Applications of first and higher derivatives of the energies obtained by molecular orbital (MO) calculations have revolutionized computational chemistry, allowing molecular structures and properties to be computed efficiently and reliably [1-5]. Almost all electronic structure codes compute analytic first derivatives of the energy, and Pulay s paper was the first to describe a practical calculational approach. 

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