Gaussian 03 Online Manual
The cutoffs used in computing and storing integrals and the convergence criteria applied in SCF and CPHF calculations are appropriate for most molecules and basis sets. However, if a nearly linearly dependent basis set is used, very large MO coefficients may occur and in combination with the finite accuracy of other terms lead to substantial numerical errors. By default CPHF and post-SCF calculations are aborted if any MO coefficient is larger than 1000. (Note that this corresponds to a coefficient of 1012 for the contribution of an AO integral to an MO integral involving four virtual orbitals.) The NoTestMO keyword suppresses this check. It should be used only after careful thought. TestMO is the default.