The lithium–thiophene interaction: a critical study using highly correlated electronic structure approaches of quantum chemistry

Abstract

The fundamental multicentric interaction of a lithium atom with a single thiophene ring is addressed. A systematic study of
the interaction energy (IE) and geometry for the Li–T charge-transfer complex is done at the MP2 and CCSD(T) levels using
increasingly large basis sets up to aug-cc-pVQZ (AVQZ). Basis set superposition errors (BSSE) are evaluated and shown to have
a major impact on the value of the IE. The Fixed-Node Diffusion Monte Carlo (FN-DMC) method is used as an alternative basis-set-free
approach to obtain what is likely to be the most accurate estimate of the IE obtained so far. While counterpoise-corrected
MP2/AVQZ and CCSD(T)/AVTZ interaction energies are found to be −3.8 and −7.5kcal/mol, the FN-DMC method yields +1.3±1.7kcal/mol.
The slow convergence of the ab initio IE (and some key structural parameters) with respect to basis set quality and the discrepancy
with the FN-DMC result is discussed. A visualization of the electron pairing using the electron pair localization function
(EPLF) for the Li-doped versus undoped thiophene is also presented.

KeywordsConjugated organic polymers-Polythiophene-Plastic electronics-Quantum Monte Carlo-FN-DMC-CCSD(T)-EPLF