Warning messages:
1: In vcov.merMod(object, use.hessian = use.hessian) :
  variance-covariance matrix computed from finite-difference Hessian is
not positive definite or contains NA values: falling back to var-cov estimated from RX
2: In vcov.merMod(object, correlation = correlation, sigm = sig) :
  variance-covariance matrix computed from finite-difference Hessian is
not positive definite or contains NA values: falling back to var-cov estimated from RX
  这是运行以下程序出来，大概提示应该是 方差协方差矩阵 那里出问题了，请问用nlemr()程序时，应该用什么选项来指定我要的方差协方差矩阵呢。