Experiment with
Ps=[100:3.447:1000];
Ps(end)=PU;
P_V=[];
parfor j=1:length(Ps)
MsC = Mstructure;
MsC.p=Ps(j);
[V] =f(MsC);
P_V=[P_V; Ps(j) V];
end
That should get rid of the error about the way Mstructure is defined.
However, you will still have problems.
Remember that parfor explicitly does not execute the iterations in strictly increasing order, so iteration #262 will almost certainly run before iteration #1 does. That is a problem for you because you are trying to append results on to the end of P_V in the order the results are produced, but the order is going to be irregular.
You indicate that your existing code works as a loop. It can only work if the value returned by f is a scalar or row vector. If the length of results expected is L, then change
P_V=[];
to
P_V = zeros(length(Ps), L);
and change
P_V=[P_V; Ps(j) V];
to
P_V(j, :) = [Ps(j), V];
