Mode_compute = 6 and log likelihood

Hi all, I have a question. Is it possible that there exists a relation between the use of mode_compute = 6 in the dsge-var and the outcoming log likelihood being positive? (and posterior also)?
Another question is: how can I choose other starting values for parameters in order to use another mode_compute without getting the error: ??? Error using ==> chol… ???
Many thanks.