First Principle Calculations of the Potential Energy Curves for Electronic States of the Lithium Dimer

Abstract

A multireference coupled cluster (CC) approach formulated in the (2,0) sector of the Fock space (FS) is applied to study electronic states of the Li2 molecule. The CC model including single (S) and double (D) excitations from the reference configuration is considered. The FS-CCSD(2,0) method is applicable to the description of the double electron attached states, which implies that in the neutral molecule studies the doubly ionized reference should be adopted. The results of this study include potential energy curves (PECs) and selected spectroscopic constants for 34 electronic states correlating to five lowest dissociation limits of Li2. Both the PECs and the computed spectroscopic constants agree very well with the experimental data wherever the latter are available. This indicates that the rigorous ab initio method can be successfully applied to study the energetics and molecular properties of the alkali metal diatomics.

Accurate potential energy curves of Li2 and LiH: A Quantum Monte-Carlo (QMC) study

“…In addition, Figure 2 shows computed PEC by the present paper and some of the obtained curves by the other researches [23,26,36]. Also, Reynolds et al [37] computed a curve for lithium dimer with DMC method.…”

“…For the triplet state , a value of 0.0414 has been obtained by Linton et al [42]. In addition to experimental measurements, many quantum chemists have investigated lithium dimer molecule by different methods such as MCSCF, MCCI and CC [7,26,36,43,44]. Table 4 gives some of the computed results of the present and previous works.…”

“…In addition, the correlation energy of Li 2 molecule is shown in Table 1. The DMC methods calculated more than 98.5% of the correlation energy while a post-HF method like multireference coupled cluster (CC) obtained about 93.5% of the correlation energy [36].…”

“…Previous studies claim that more accurate methods such as ECG or CC are more efficient than a QMC computation for small molecules like Li 2 or LiH molecules [36]. In post-HF methods, however, basis set selection is more important and delicate, especially for computing PEC.…”

“…Sometimes, setting these parameters correctly, are the most time consuming part of the computation. For example, in the previous works, model space size is more than thousands [7,36]. In contrast to post-HF methods, in the QMC method the initial PEC is constructed by a single determinant from which the required wave functions for the curve are obtained.…”