Add option to include spin-magnitude term in Hamiltonian

PiperOrigin-RevId: 643060421
Change-Id: If7d0779f080ccf3bfdcd5a583dc8178d70fd5234

ferminet/base_config.py

@@ -80,6 +80,10 @@ def default() -> ml_collections.ConfigDict:
           # If using Wasserstein QMC, this parameter controls the amount of
           # "default" VMC gradient to mix in. Otherwise, it is ignored.
           'vmc_weight': 0.0,
+          # If nonzero, add a term to the Hamiltonian proportional to the spin
+          # magnitude. Useful for removing non-singlet states from excited
+          # state calculations.
+          'spin_energy': 0.0,
           # KFAC hyperparameters. See KFAC documentation for details.
           'kfac': {
               'invert_every': 1,

ferminet/tests/train_test.py

@@ -113,6 +113,24 @@ def test_training_step(self, system, optimizer, complex_, states, laplacian):
     # ensure they actually run without a top-level error.
     train.train(cfg)
 
+  @parameterized.parameters([{'states': 0}, {'states': 3}])
+  def test_s2_energy(self, states):
+    cfg = diatomic.get_config()
+    cfg.system.molecule_name = 'LiH'
+    cfg.network.ferminet.hidden_dims = ((16, 4),) * 2
+    cfg.network.determinants = 2
+    cfg.batch_size = 32
+    cfg.system.states = states
+    cfg.pretrain.iterations = 10
+    cfg.mcmc.burn_in = 10
+    cfg.optim.iterations = 3
+    cfg.optim.spin_energy = 1.0
+    cfg.log.save_path = self.create_tempdir().full_path
+    cfg = base_config.resolve(cfg)
+    # Calculation is too small to test the results for accuracy. Test just to
+    # ensure they actually run without a top-level error.
+    train.train(cfg)
+
   def test_random_pretraining(self):
     cfg = diatomic.get_config()
     cfg.system.molecule_name = 'LiH'

ferminet/train.py

@@ -696,7 +696,7 @@ def log_network(*args, **kwargs):
         cfg.system.make_local_energy_fn.rsplit('.', maxsplit=1))
     local_energy_module = importlib.import_module(local_energy_module)
     make_local_energy = getattr(local_energy_module, local_energy_fn)  # type: hamiltonian.MakeLocalEnergy
-    local_energy = make_local_energy(
+    local_energy_fn = make_local_energy(
         f=signed_network,
         charges=charges,
         nspins=nspins,
@@ -705,7 +705,7 @@ def log_network(*args, **kwargs):
         **cfg.system.make_local_energy_kwargs)
   else:
     pp_symbols = cfg.system.get('pp', {'symbols': None}).get('symbols')
-    local_energy = hamiltonian.local_energy(
+    local_energy_fn = hamiltonian.local_energy(
         f=signed_network,
         charges=charges,
         nspins=nspins,
@@ -715,6 +715,29 @@ def log_network(*args, **kwargs):
         states=cfg.system.get('states', 0),
         pp_type=cfg.system.get('pp', {'type': 'ccecp'}).get('type'),
         pp_symbols=pp_symbols if cfg.system.get('use_pp') else None)
+
+  if cfg.optim.get('spin_energy', 0.0) > 0.0:
+    # Minimize <H + c * S^2> instead of just <H>
+    # Create a new local_energy function that takes the weighted sum of
+    # the local energy and the local spin magnitude.
+    local_s2_fn = observables.make_s2(
+        signed_network,
+        nspins=nspins,
+        states=cfg.system.states)
+    def local_energy_and_s2_fn(params, keys, data):
+      local_energy, aux_data = local_energy_fn(params, keys, data)
+      s2 = local_s2_fn(params, data, None)
+      weight = cfg.optim.get('spin_energy', 0.0)
+      if cfg.system.states:
+        aux_data = aux_data + weight * s2
+        local_energy_and_s2 = local_energy + weight * jnp.trace(s2)
+      else:
+        local_energy_and_s2 = local_energy + weight * s2
+      return local_energy_and_s2, aux_data
+    local_energy = local_energy_and_s2_fn
+  else:
+    local_energy = local_energy_fn
+
   if cfg.optim.objective == 'vmc':
     evaluate_loss = qmc_loss_functions.make_loss(
         log_network if use_complex else logabs_network,