OK Kit, I agree with your comment. But I still don't understand why I haven't the same error message doing "eret post" respectively with (b1,V1) and (Sameb1, SameV1) while V1 and SameV1 are the same (and have the same determinant)...
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Kit said
Well, VCEs are supposed to be positive definite matrices. PD mat have positive determinants that are not too close to zero.
If Stata's best guess about this matrix is that its determinant is a negative number very close to zero, the error message is well warranted. The problem is not with 'ereturn post'; it is with what you are trying to tell Stata is a valid VCE.
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