Selected Correlation Energy Calculations on Polymers

Ground-State Energy of an Infinite Metailic Hydrogen Chain 

In the case of an equidistant (metallic) H-atom chain, the BZ was divided into two parts in each of which the coefficients were expressed as a three-term trigonometric expansion [see equation (5.48) and the paper by Suhai und Ladild ]. 

Five different basis sets were used for the calculation at both the RHF and correlated levels, as shown in Tables 5.1 and 5.2, where the notation msins means that on each hydrogen atom m 5-type Gaussians are centered and are divided into n groups (contractions). The exponents and contraction coefficients of the first two sets were taken from the literature, and the exponents of the six 5-type Gaussians from another work. In the fourth basis set these functions were supplemented by two 5-type bond functions (2 ) centered in the middle between the hydrogen atoms, while in the fifth basis a set of three p-type atomic polarization functions p, Py, Pz) was used. The contraction coefficients and the exponents of the bond and polarization functions were optimized previously. 

The quality of these basis sets can be demonstrated by the example of the H2 molecule. The HF energy extrapolated to the complete basis set is hf = 30.847 eV at the optimal interatomic distance of R = 1.386 au. Suppose, for instance, that the fifth basis set in Table 5.1 is used, then we obtain hf= 30.814 eV and R = 1.398 au. Application of second-order Moller-Plesset theory yields for hydrogen in this basis set a 

TABLE 5.1. Some Physical Properties of the Infinite Metallic Hydrogen Chain Obtained by the RHF Method Using Different Atomic Basis Sets  

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