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//! ReaxFF parameter set and parsing.
use serde::{Deserialize, Serialize};
/// Parameters for a single atom type in ReaxFF.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct ReaxffAtomParams {
/// Atomic number.
pub element: u8,
/// σ bond equilibrium distance (Å).
pub r_sigma: f64,
/// π bond equilibrium distance (Å).
pub r_pi: f64,
/// ππ bond equilibrium distance (Å).
pub r_pipi: f64,
/// Bond order parameters.
pub p_bo1: f64,
pub p_bo2: f64,
pub p_bo3: f64,
pub p_bo4: f64,
pub p_bo5: f64,
pub p_bo6: f64,
/// Valence (nominal number of bonds).
pub valence: f64,
}
/// Full ReaxFF parameter set.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct ReaxffParams {
/// Per-atom parameters.
pub atom_params: Vec<ReaxffAtomParams>,
/// Bond dissociation energy parameters.
pub bond_de: Vec<Vec<f64>>,
/// Bond energy parameters.
pub p_be1: f64,
pub p_be2: f64,
/// Over-coordination penalty.
pub p_ovun1: f64,
/// Over-coordination threshold.
pub p_val3: f64,
/// Under-coordination penalty.
pub p_ovun5: f64,
/// Equilibrium angles (radians) [i][j][k].
pub equilibrium_angles: Vec<f64>,
/// Angle force constants.
pub angle_force_constants: Vec<f64>,
/// Non-bonded cutoff (Å).
pub cutoff: f64,
}
impl ReaxffParams {
/// Build default C/H/O/N parameters for ReaxFF.
pub fn default_chon() -> Self {
let atom_params = vec![
// H
ReaxffAtomParams {
element: 1,
r_sigma: 0.656,
r_pi: 0.0,
r_pipi: 0.0,
p_bo1: -0.0500,
p_bo2: 6.9136,
p_bo3: 0.0,
p_bo4: 6.0,
p_bo5: 0.0,
p_bo6: 6.0,
valence: 1.0,
},
// C
ReaxffAtomParams {
element: 6,
r_sigma: 1.3825,
r_pi: 1.1359,
r_pipi: 1.2104,
p_bo1: -0.0777,
p_bo2: 6.7268,
p_bo3: -0.1000,
p_bo4: 9.1628,
p_bo5: -0.1418,
p_bo6: 13.3056,
valence: 4.0,
},
// N
ReaxffAtomParams {
element: 7,
r_sigma: 1.2380,
r_pi: 1.1748,
r_pipi: 1.0630,
p_bo1: -0.1000,
p_bo2: 6.8773,
p_bo3: -0.1193,
p_bo4: 7.8431,
p_bo5: -0.1418,
p_bo6: 13.1260,
valence: 3.0,
},
// O
ReaxffAtomParams {
element: 8,
r_sigma: 1.2477,
r_pi: 1.0863,
r_pipi: 0.0,
p_bo1: -0.1363,
p_bo2: 5.6346,
p_bo3: -0.1743,
p_bo4: 7.6279,
p_bo5: 0.0,
p_bo6: 6.0,
valence: 2.0,
},
];
// Build bond DE matrix (simplified)
let n = atom_params.len();
let mut bond_de = vec![vec![100.0; n]; n]; // kcal/mol
// H-H, C-C, etc.
bond_de[0][0] = 104.2; // H-H
bond_de[1][1] = 145.0; // C-C
bond_de[0][1] = 99.0; // H-C
bond_de[1][0] = 99.0;
bond_de[0][3] = 111.0; // H-O
bond_de[3][0] = 111.0;
bond_de[1][3] = 85.0; // C-O
bond_de[3][1] = 85.0;
bond_de[1][2] = 73.0; // C-N
bond_de[2][1] = 73.0;
Self {
atom_params,
bond_de,
p_be1: -0.2,
p_be2: 6.25,
p_ovun1: 50.0,
p_val3: 3.0,
p_ovun5: 10.0,
equilibrium_angles: vec![
std::f64::consts::FRAC_PI_2 * 2.0 / 3.0 * std::f64::consts::PI;
100
],
angle_force_constants: vec![50.0; 100],
cutoff: 10.0,
}
}
/// Get bond dissociation energy for pair (i, j) by atom index.
pub fn get_bond_de(&self, i: usize, j: usize) -> f64 {
if i < self.bond_de.len() && j < self.bond_de[i].len() {
self.bond_de[i][j]
} else {
100.0 // default
}
}
/// Get equilibrium angle for triplet i-j-k.
pub fn get_equilibrium_angle(&self, _i: usize, _j: usize, _k: usize) -> f64 {
109.47_f64.to_radians() // tetrahedral default
}
/// Get angle force constant for triplet i-j-k.
pub fn get_angle_force_constant(&self, _i: usize, _j: usize, _k: usize) -> f64 {
50.0 // kcal/mol/rad²
}
/// Map element to parameter index.
pub fn element_index(&self, z: u8) -> Option<usize> {
self.atom_params.iter().position(|p| p.element == z)
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn default_chon_has_four_elements() {
let params = ReaxffParams::default_chon();
assert_eq!(params.atom_params.len(), 4);
}
#[test]
fn element_index_finds_carbon() {
let params = ReaxffParams::default_chon();
assert!(params.element_index(6).is_some(), "carbon should be found");
}
#[test]
fn element_index_none_for_missing() {
let params = ReaxffParams::default_chon();
assert!(
params.element_index(79).is_none(),
"gold should not be in CHON"
);
}
#[test]
fn bond_de_is_positive() {
let params = ReaxffParams::default_chon();
let de = params.get_bond_de(0, 1);
assert!(de > 0.0, "bond dissociation energy should be positive");
}
#[test]
fn equilibrium_angle_is_tetrahedral() {
let params = ReaxffParams::default_chon();
let angle = params.get_equilibrium_angle(0, 1, 2);
let tetra = 109.47_f64.to_radians();
assert!((angle - tetra).abs() < 0.01);
}
}