Exploring the accuracy of the equation-of-motion coupled-cluster band gap of solids

While the periodic equation-of-motion coupled-cluster (EOM-CC) method promises systematic improvement of electronic band gap calculations in solids, its practical application at the singles and doubles level (EOM-CCSD) is hindered by severe finite-size errors in feasible simulation cells. We present a hybrid approach combining EOM-CCSD with the computationally less demanding <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mi>G</a:mi><a:mi>W</a:mi></a:mrow></a:math> approximation to estimate thermodynamic limit band gaps for several insulators and semiconductors. Our method substantially reduces required cell sizes while maintaining accuracy. Comparisons with experimental gaps and self-consistent <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mrow><b:mi>G</b:mi><b:mi>W</b:mi></b:mrow></b:math> calculations reveal that deviations in EOM-CCSD predictions correlate with reduced single excitation character of the excited many-electron states. Our work not only provides a computationally tractable approach to EOM-CC calculations in solids but also reveals fundamental insights into the role of single excitations in electronic-structure theory.