  Two new schemes for computing molecular total atomization energies (TAEs)
and/or heats of formation ($\Delta H^\circ_f$) of first-and second-row
compounds to very high accuracy are presented. The more affordable scheme, W1
(Weizmann-1) theory, yields a mean absolute error of 0.30 kcal/mol and includes
only a single, molecule-independent, empirical parameter. It requires CCSD
(coupled cluster with all single and double substitutions) calculations in
$spdf$ and $spdfg$ basis sets, while CCSD(T) [i.e. CCSD with a
quasiperturbative treatment of connected triple excitations] calculations are
only required in $spd$ and $spdf$ basis sets. On workstation computers and
using conventional coupled cluster algorithms, systems as large as benzene can
be treated, while larger systems are feasible using direct coupled cluster
methods. The more rigorous scheme, W2 (Weizmann-2) theory, contains no
empirical parameters at all and yields a mean absolute error of 0.23 kcal/mol,
which is lowered to 0.18 kcal/mol for molecules dominated by dynamical
correlation. It involves CCSD calculations in $spdfg$ and $spdfgh$ basis sets
and CCSD(T) calculations in $spdf$ and $spdfg$ basis sets. On workstation
computers, molecules with up to three heavy atoms can be treated using
conventional coupled cluster algorithms, while larger systems can still be
treated using a direct CCSD code. Both schemes include corrections for scalar
relativistic effects, which are found to be vital for accurate results on
second-row compounds.
