The van der Waals complex between the Cl(P-2) atom and the HCl molecule was carefully investigated. Using high-level ab initio molecular orbital theory with extended basis sets, we analyse the effects of level of calculation, spin projection, basis set superposition error (BSSE), spin-orbit coupling, and harmonic zero-point energy (ZPE) on the energy changes. The non-collinear approach (1(2)A' ground state) shows a deeper well at 0 K, 401 cm(-1), than the collinear approach ((2)Pi ground state), 329 cm(-1). It is concluded that these results represent the balance of a complicated interplay of many factors, which are affected by uncertainties in the theoretical calculations. (C) 2003 Published by Elsevier B.V.