Exploring the accuracy of relative molecular energies with local correlation theory

Local coupled-cluster singles-doubles theory (LCCSD) is a theorist's attempt to capture electron-electron correlation in a fast amount of time and with chemical accuracy. Many of the difficult computational hurdles have been navigated over the last twenty years, including how to construct a linear scaling algorithm and how to produce smooth potential energy surfaces. Nevertheless, there remains the question of just how accurate a local correlation model can be, and what are the chemical limits within which local models are largely applicable. Here, we investigate how accurately can LCCSD approximate full CCSD for cases of atomization energies, isomerization energies, conformational energies, barrier heights and electron affinities. Our conclusion is that LCCSD computes relative energies that are correct to within 1-2 kcal mol^-1 of the CCSD energy using relatively aggressive cutoffs and over a broad range of different molecular environments - alkane isomers, dipeptide conformations, Diels-Alder transition states and electron attachment in charge delocalized systems. These findings should push the reach of local correlation applications into new research terrain, including molecules on metal cluster surfaces or perhaps even metal-molecule-metal clusters.