How to formulate a non convex equality constraint for fmincon?

I have an equality constraint that needs to be formulated into fmincon.

v'*Mp*v - v'*Mn*v - Pf = 0.

where Pf is a scalar variable, v is a variable vector, Mp and Mn are symmetric positive semi-definite matrices but -v'*Mn*v is a concave function. As a result, formulating the above equation as it is isn't working.

How do I formulate this constraint in MATLAB for fmincon?

[Torsten]

"fmincon" does not take care of concavity or convexity of the functions involved. Thus I don't know why entering your formula as a nonlinear constraint in "nonlcon" should not work.

Best wishes

Torsten.