The atomization energy of Mg-4

The atomization energy of Mg-4 is determined using the MP2 and CCSD(T) leve ls of theory. Basis set incompleteness, basis set extrapolation, and core-v alence effects are discussed. Our best atomization energy, including the ze ro-point energy and scalar relativistic effects, is 24.6 +/- 1.6 kcal/mol. Our computed and extrapolated values are compared with previous results, wh ere it is observed that our extrapolated MP2 value is in good agreement wit h the MP2-R12 value. The CCSD(T) and MP2 core effects are found to have the opposite signs. (C) 1999 Elsevier Science B.V. All rights reserved.