R help - Jul 2006 - Tobit variance covariance matrix

Hi,

How can I recover the variance-covariance matrix of the tobit model from 
the variance-covariance of the survreg?
I first used to the survreg function and then I selected the variance 
matrix. However, the last parameter is log(scale) and not the variance 
of the standard deviation of the censored distribution as in the Tobit 
model.
tobit<- survreg(Surv(y, y > 0, type ='left')~ 0+ z + vh, dist =
'gaussian');
Om   <- tobit$var;

Thanks

Leandro