Determinant of Hessian matrix



 Dear netters,
 I have tried to evaluate the determinant of Hessian matrix evaluated
 using Discover3.0. The Hessian is positive definite at minimun
 conformation but some determinants are negative in my cases.
 I carried out full minimization using Quasi-Newton
 or Newton-Raphson method until maximum derivative is less than 0.00001.
 For example,
 ---------------------------------------------------------
                       the determinant of Hessian matrix
                       ----------------------------------
  method               butane           alanine dipeptide
 ---------------------------------------------------------
 Quasi-Newton          negative            positive
 Newton-Raphson        positive            negative
 ---------------------------------------------------------
 Please let me know the reason.
 Thanks.
 -------------------------
 moon - at - kistmail.kist.re.kr.