Present investigation reveals that diffuse electron compounds M2X predict large (10(6) a.u.) gamma(av) value. The basis set dependence of gamma(av) showed that the quality of basis function rather than its size is more important. Sadlej's Pol basis set which is nearly four times less in size than aug-cc-pVQZ can give reliable estimate of gamma(cav). MP2 method in conjunction with large basis sets can give comparable results of gamma(av) that could be obtained from CCSD or CCSD(T). Higher order energy correction to MP2 leads to oscillatory behavior but inclusion of triple excitations to CCSD improves the magnitude of gamma(av). (C) 2017 Elsevier B.V. All rights reserved.