138 lines
5.0 KiB
Python
138 lines
5.0 KiB
Python
#!/usr/bin/env python
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import numpy as np
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from jax import numpy as jnp, jit, grad
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# toy moleculat dynamic system parameters (constants)
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De = 1.6 # [eV (electronvolt)]
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alpha = 3.028 # [A^-1]
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re = 1.411 # [A]
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mass = 18.998403 # atomic mass of a single particle
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def dump(file, position, velocity, box_size, mode = "w", comment = ""):
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"""
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Dumps `position` and `velocity` to `file` with meta data.
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First writes a header to `file` in `mode` file write mode defines as three
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lines.
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`<nr of particles [int]>`
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`<comment (ignored)>`
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`<box side lenght [float]>`
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then each line (nr of particles many) have the form `x y z vx vy vx`.
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"""
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header = [str(position.shape[0]), comment, str(box_size)]
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data = np.hstack((position, velocity))
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np.savetxt(file, data, comments = "", header = "\n".join(header))
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def load(file):
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"""
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Loads a state from `file` and returns the `position` and `velocity` system
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states as two `numpy` arrays.
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As a side effect the config object as agumented with `file` header info.
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"""
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if isinstance(file, str):
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with open(file) as handle:
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return load(handle)
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# read header
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nr_particles = int(file.readline().strip())
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file.readline() # ignore comment line
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box_size = float(file.readline().strip())
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# read the state data, each line constains `x y z vx vy vz`
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data = np.loadtxt(file, max_rows = nr_particles)
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return np.hsplit(data, 2), box_size
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def iter_load(file):
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"""
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Iterator yielding similar to `load` entries from `file` in the same format.
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"""
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if isinstance(file, str):
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with open(file) as handle:
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yield from iter_load(handle)
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else:
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while True:
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# read header
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line = file.readline().strip()
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if not line:
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return
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nr_particles = int(line)
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file.readline() # ignore comment line
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line = file.readline().strip()
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if not line:
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return
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box_size = float(line)
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# read the state data, each line constains `x y z vx vy vz`
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data = np.loadtxt(file, max_rows = nr_particles)
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if not data.shape[0] == nr_particles:
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return
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yield np.hsplit(data, 2), box_size
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def print_prog_bar(index, max_index):
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"""
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Simple progress bar. Just for convenience.
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"""
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if (index % 100) and (max_index != index + 1):
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return
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progress = (30 * (index + 1)) // max_index
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print(f"\r{(index + 1)}/{max_index} - " \
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f"[{'':=>{(progress)}}{'': >{(30 - progress)}}]",
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end = "\n" if max_index == index + 1 else "",
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flush = True)
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@jit
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def energy(position, box_size):
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"""
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Computes the potential energy of a system of particles with `position` using
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Morses potential.
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"""
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# enforce expected shape
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# (`scipy.optimize.minimize` drops shape information)
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if len(position.shape) == 1:
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position = position.reshape((position.shape[0] // 3, 3))
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# compute all pairwise position differences (all!)
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diff = position[:, jnp.newaxis, :] - position
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# extract only one of two distance combinations of non-equal particles
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lower_tri = jnp.tril_indices(position.shape[0], k = -1)
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diff = diff[lower_tri[0], lower_tri[1], :]
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# fold space in all directions. The `max_dist` is the maximum distance
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# in the folded space untill the distance through the folded space
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# border is smaller than inside the box. 3-axis parallel fold
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max_dist = box_size / 2.0
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diff = jnp.mod(diff + max_dist, box_size) - max_dist
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# Compute distances
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dist = jnp.linalg.norm(diff, axis = 1)
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# calc inbetween exp(.) expression
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ex = jnp.exp(-alpha * (dist - re))
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# evaluate Morse potential
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return De * jnp.sum(ex * (ex - 2.0))
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@jit
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def force(position, box_size):
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"""
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Computes the forces acting on each particle in `position` as the negative
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gradient of the potential energy.
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"""
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return -grad(energy)(position, box_size)
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@jit
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def kinetic(velocity):
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"""
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Computes the kenetic energy given a systems `velocity` state.
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"""
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return (mass / 2.0) * (velocity**2).sum()
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@jit
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def step(position, velocity, acceleration, box_size, step_size):
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"""
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Performs a single Newton time step with `step_size` given system state
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through the current particle `position`, `velocity` and `acceleration`.
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"""
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# update position with a second order Taylor expantion
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position += step_size * velocity + (0.5 * step_size**2) * acceleration
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# compute new particle acceleration through Newton’s second law of motion
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acceleration_next = force(position, box_size) / mass
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# update velocity with a finite mean approximation
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velocity += (0.5 * step_size) * (acceleration + acceleration_next)
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# updated state
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return position, velocity, acceleration_next
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