neural ode added

jaxpm/pm.py

@@ -94,3 +94,39 @@ def pgd_correction(pos, params):
     dpos_pgd = forces_pgd*alpha
    
     return dpos_pgd
+
+
+def make_neural_ode_fn(model, mesh_shape):
+    def neural_nbody_ode(state, a, cosmo, params):
+        """
+        state is a tuple (position, velocities)
+        """
+        pos, vel = state
+        kvec = fftk(mesh_shape)
+
+        delta = cic_paint(jnp.zeros(mesh_shape), pos)
+
+        delta_k = jnp.fft.rfftn(delta)
+
+        # Computes gravitational potential
+        pot_k = delta_k * laplace_kernel(kvec) * longrange_kernel(kvec, r_split=0)
+
+        # Apply a correction filter
+        kk = jnp.sqrt(sum((ki/jnp.pi)**2 for ki in kvec))
+        pot_k = pot_k *(1. + model.apply(params, kk, jnp.atleast_1d(a)))
+
+        # Computes gravitational forces
+        forces = jnp.stack([cic_read(jnp.fft.irfftn(gradient_kernel(kvec, i)*pot_k), pos) 
+                          for i in range(3)],axis=-1)
+
+        forces = forces * 1.5 * cosmo.Omega_m
+
+        # Computes the update of position (drift)
+        dpos = 1. / (a**3 * jnp.sqrt(jc.background.Esqr(cosmo, a))) * vel
+
+        # Computes the update of velocity (kick)
+        dvel = 1. / (a**2 * jnp.sqrt(jc.background.Esqr(cosmo, a))) * forces
+
+        return dpos, dvel
+    return neural_nbody_ode
+