I don't know the details of your problem, so I don't know what you need that inverse for. Assuming you really need to see the individual values of that matrix, I guess you can solve the problem by equations. Something along these lines

model ComplexInverse
  constant Complex j = Complex(0,1);
  Complex A[2,2] = {{1.0+0*j, 1.0+j}, {0.0 + 0*j, 1.0 + 0*j}};
  Complex I[2,2];
  Complex invA[2,2];
equation
  I = identity(2)*Complex(1.0,0.0);
  A*invA = I;
end ComplexInverse;

That's wonderful, Francesco.
Sometimes I forget that I'm playing with a very powerful programming language!
This helps a lot solving my problem.

In my original idea I had to invert a matrix inside a function, so inside an algorithm section; this in principle forbids the use of equations, and the technique you proposed.

But my function has to be called only at the beginning of the simulation, so I will try to mix the equation required for the inversion technique you suggested with equations using function calls, in an "when initial()" section.

Two further remarks

1. I would have liked to use parameters with fixed = false and initial equations, but due to some OMC bug this didn't work, so I used plain variables for my example.

2. Even if you do so, OMC will recognise that these variables are not time-varying, and move their computation to the initialization section automatically