simple example

examples/README.md

@@ -1,5 +1,7 @@
 # MPS-FQE Examples
 
+- fqe_to_mps.py
+  - Calculates the expectation value of a sparse operator with FQE and MPS-FQE with bra = ket and bra != ket.
 - N2_asp.py
   - Performs adiabatic state preparation on N<sub>2</sub> in a minimal basis performed with exact statevector evolution and TD-DMRG under truncated bond dimension.
 - N2_imag.py
@@ -9,20 +11,6 @@
 - adapt.py
   - Performs an ADAPT-VQE simulation on a 4-member Hydrogen chain under truncated bond dimension using RK4 propagation.
 
-
-## Collaborators
-__QSimulate__:\
-Alec F. White, Justin Provazza, Klaas Gunst
-
-__Google__:\
-Nicholas C. Rubin
-
-__California Institute of Technology__:\
-Huanchen Zhai
-
-## How to cite
-When using MPS-FQE for research projects, please cite:
-
 ```
 @article{mps_fqe_2023,
     author       = {Justin Provazza, Klaas Gunst, Huanchen Zhai, Garnet K.-L. Chan,

examples/fqe_to_mps.py

@@ -0,0 +1,45 @@
+import fqe
+from openfermion import FermionOperator
+from openfermion.utils import hermitian_conjugated
+from mps_fqe.wavefunction import MPSWavefunction
+
+
+if __name__ == '__main__':
+    # System parameters
+    nele = 4
+    sz = 0
+    norbs = 4
+    # MPS parameters
+    mbd = 20
+    cutoff = 1e-14
+
+    # Generate random FQE wavefunction instance
+    fqe_wfn = fqe.Wavefunction([[nele, sz, norbs]])
+    fqe_wfn.set_wfn(strategy="random")
+    # Generate MPS-FQE wavefunction instance from FQE wavefunction
+    mps_wfn = MPSWavefunction.from_fqe_wavefunction(fqe_wfn,
+                                                    max_bond_dim=mbd,
+                                                    cutoff=cutoff)
+
+    # Generate Openfermion operator
+    of_operator = FermionOperator("1^ 3")
+    of_operator += hermitian_conjugated(of_operator)
+    # Use it to generate FQE operator
+    fqe_operator = fqe.sparse_hamiltonian.SparseHamiltonian(of_operator)
+
+    # Expectation value
+    print("<psi|o|psi>:\nFQE:{}\nMPS-FQE:{}".format(
+        fqe_wfn.expectationValue(fqe_operator),
+        mps_wfn.expectationValue(fqe_operator))
+          )
+
+    fqe_wfn_evolved = fqe_wfn.time_evolve(1, fqe_operator)
+    mps_wfn_evolved = mps_wfn.time_evolve(1, fqe_operator)
+
+    # Bra state not equal to ket state
+    print("<psi|o exp(-i o)|psi>:\nFQE:{}\nMPS-FQE:{}".format(
+        fqe_wfn_evolved.expectationValue(fqe_operator,
+                                         brawfn=fqe_wfn),
+        mps_wfn_evolved.expectationValue(fqe_operator,
+                                         brawfn=mps_wfn))
+          )