A general question – why should some versions of mode_compute work while others result in an error about not having a positive definite matrix? For my code, for example, mode_compute=6 works while 4, 8, 9, and 10 do not. Is this an issue?
In addition, is there any way to specify the starting jscale value for mode_compute=6?
It generally means you have not found a true mode (characterized by a semi-definite Hessian). This is an issue if you do not run your chain until convergence.