# How to set an initial guess for the iterative solver in Comsol?

How to set the initial guess for the iterative solver GMRES or FGMRES for linear problems (Helmholtz equation of RF module) in Comsol?

1. It is a common practice to use a solution with the preconditioner as an initial guess. So, say, you are solving a system $$Ax=b \implies P^{-1}Ax=P^{-1}b \tag{1} \label{1}$$ with a left preconditioner $$P$$. Then, the solution of $$\eqref{2}$$ $$Px=b \tag{2} \label{2}$$ will serve as an initial guess for $$\eqref{1}$$. So, if a preconditioner is used, usually, one does not start with random/zero initial guess, and the quality of the initial guess is directly related to the quality of the preconditioner $$P$$.