Analysis of the errors in explicitly correlated electronic structure theory

The explicitly correlated second order Moller - Plesset (MP2- R12) methods perform well in reproducing the last detail of the correlation cusp, allowing higher accuracy than can be accessed through conventional means. Nevertheless in basis sets that are practical for calculations on larger systems (i.e., around triple- or perhaps quadruple- zeta) MP2- R12 fails to bridge the divide between conventional MP2 and the MP2 basis set limit. In this contribution we analyse the sources of error in MP2- R12 calculations in such basis sets. We conclude that the main source of error is the choice of the correlation factor r(12). Sources of error that must be avoided for accurate quantum chemistry include the neglect of exchange commutators and the extended Brillouin condition. The generalized Brillouin condition is found not to lead to significant errors.