how to re-calculate error matrix in TMinuit

Dear Rooter,

I have the following question about TMinuit:

I have performed a Maximum Likelihood fit using TMinuit, and obtained estimated parameters and errors.
Then I want to get a new error matrix by changing some variables (such as weight) at the found minimum of the -log likelihood function. How can I do this without performing a new minimization procedure of TMinuit ?

Thanks in advance !

Best Regards,
Xianguo

-- 
Take flight into the sky, beyond the moon, beyond my mind.

Xianguo LU
Physics Department, Peking University
Beijing, China
Tel: 0086-10-62753888(o)