Log-likelihood calculation in lme

On a real data set, running the lme function, I get
parameters estimation and a log-likelihood value.
Nevertheless, the variance-covariance matrix in this
case had a determinant close to zero. So, I could not
calculate the log-likelihood myself with the classical
expression.
What is the calculus made in lme?
Thanks for all suggestions.

Tessa P-J
(student)