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Operator Matrix Elements Involving Two MPS

Say we have two MPS wavefunctions, @@|\psi\!\rangle@@ and @@|\phi\rangle@@ , and that we wish to compute @@\langle\phi|O_i O_j|\psi\rangle@@ for some local operators @@O_i@@ and @@O_j@@ . We can do this as follows:

int i = 4;
int j = 10;
auto op_i = sites.op("Sz",i);
auto op_j = sites.op("Sz",j);

auto M = psi.A(1)*dag(prime(phi.A(1),Link));
for(auto n : range1(2,N))
    M *= psi.A(n);
    if(n == i)
        M *= op_i*dag(prime(phi.A(i)));
    else if(n == j)
        M *= op_j*dag(prime(phi.A(j)));
        M *= dag(prime(phi.A(n),Link));
auto result = M.real();

Note: if @@i@@ or @@j@@ are equal to 1, one must modify the line in the code above where @@M@@ is first defined. Note: if a complex valued result is expected, change the last line to use M.cplx();.

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