Merge pull request #38 from markusschmitt/Zero-Magnetization-Sampler

Zero-Magnetization Sampler

jVMC/sampler.py

@@ -39,6 +39,30 @@ def propose_spin_flip_Z2(key, s, info):
     doFlip = random.randint(flipKey, (1,), 0, 5)[0]
     return jax.lax.cond(doFlip == 0, lambda x: 1 - x, lambda x: x, s)
 
+def propose_spin_flip_zeroMag(key, s, info):
+    # propose spin flips that stay in the zero magnetization sector
+
+    idxKeyUp, idxKeyDown, flipKey = jax.random.split(key, num=3)
+
+    # can't use jnp.where because then it is not jit-compilable
+    # find indices based on cumsum
+    bound_up = jax.random.randint(idxKeyUp, (1,), 1, s.shape[0] * s.shape[1] / 2 + 1)
+    bound_down = jax.random.randint(idxKeyDown, (1,), 1, s.shape[0] * s.shape[1] / 2 + 1)
+
+    id_up = jnp.searchsorted(jnp.cumsum(s), bound_up)
+    id_down = jnp.searchsorted(jnp.cumsum(1 - s), bound_down)
+
+    idx_up = jnp.unravel_index(id_up, s.shape)
+    idx_down = jnp.unravel_index(id_down, s.shape)
+
+    s = s.at[idx_up[0], idx_up[1]].set(0)
+    s = s.at[idx_down[0], idx_down[1]].set(1)
+
+    # On average, do a global spin flip every 30 updates to
+    # reflect Z_2 symmetry
+    doFlip = random.randint(flipKey, (1,), 0, 5)[0]
+    return jax.lax.cond(doFlip == 0, lambda x: 1 - x, lambda x: x, s)
+
 
 class MCSampler:
     """A sampler class.