oopsyNo, that's another thread!
The only bad point is that the matrix A is no longer symmetric with these BCs, and I don't know what happens to monotonicity and conservation properties of the FD scheme. But the numerics seemed ok AFAIR.Yes, that was also one of my suggestions and is standard.
I started to work on it. I need a half day more to finish a first readable draft. I hope you will receive it at end of this week !Can you write down the precise 'ehancements'? e.g. for heat equation and then BS?