Diagonal orbital-invariant explicitly-correlated coupled-cluster method

We present an orbital-invariant explicitly-correlated coupled-cluster method where we determine the contribution from the geminal pair functions by the known first-order electron coalescence conditions. This method is free from geminal basis set superposition errors and yields quintuple-zeta quality correlation energies using triple-zeta orbital basis sets. (c) 2008 Elsevier B.V. All rights reserved.