Is combining meta-GGA correlation functionals with the OPTX exchange functional useful?

The recent generalized gradient approximation (GGA) density functional OCS1 of Handy and Cohen is implemented in the deMon code and tested on a carefully selected set of problems. OCS1 is found to be accurate for molecular atomization energies, transition metal-ligand bonds, and systems with intramolecular hydrogen bonds. However, OCS1 encounters problems for systems with intermolecular hydrogen bonds. It also tends to elongate bond lengths systematically, and sometimes significantly. The OPTX exchange is combined with three meta-GGA correlation functionals, Lap3, tau 1, and tau 2, the latter reported for the first time. The new meta-GGA scheme OPTX exchange plus tau 2 correlation called O tau 2 yields improved molecular geometries, NMR shielding constants, and an improved barrier height for the H+H-2 reaction. (C) 2005 Wiley Periodicals, Inc.