dynamics/

com_zero.rs

//! Zero center of mass linear drift and rotation for all atoms in the system.

use lin_alg::{f32::Vec3, f64::Vec3 as Vec3F64};

use crate::{
    MdState,
    water::{H_MASS, MASS_WATER_MOL, O_MASS},
};

const EPS: f64 = 1e-6;

impl MdState {
    /// Remove center-of-mass drift. This can help stabilize system energy.
    /// We perform the sums here as f64.
    pub fn zero_linear_momentum(&mut self) {
        let mut mass_sum = 0.0;
        let mut p_sum = Vec3F64::new_zero(); // Σ m v

        for a in &self.atoms {
            mass_sum += a.mass as f64;
            let p: Vec3F64 = (a.vel * a.mass).into();
            p_sum += p;
        }

        for w in &self.water {
            mass_sum += MASS_WATER_MOL as f64;

            let p_o: Vec3F64 = (w.o.vel * O_MASS).into();
            let p_h0: Vec3F64 = (w.h0.vel * H_MASS).into();
            let p_h1: Vec3F64 = (w.h1.vel * H_MASS).into();

            p_sum += p_o + p_h0 + p_h1;
        }

        if mass_sum <= EPS {
            return;
        }

        let vel_com: Vec3 = (p_sum / mass_sum).into();

        // Subtract uniformly so Σ m v' = 0
        for a in &mut self.atoms {
            a.vel -= vel_com;
        }

        // I don't think we need to use SHAKE/RATTLE here, as the velocity
        // change is uniform for the water atoms in a given mol.
        for w in &mut self.water {
            w.o.vel -= vel_com;
            w.h0.vel -= vel_com;
            w.h1.vel -= vel_com;
        }
    }

    // todo: Assess if you want this for multi-molecule systems.
    /// Remove rigid-body rotation.
    /// Computes ω from I ω = L about the atoms' COM, then sets v' = v - ω × (r - r_cm).
    pub fn zero_angular_momentum(&mut self) {
        let mut mass_sum = 0.0;
        let mut m_r_sum = Vec3F64::new_zero();

        for a in &self.atoms {
            mass_sum += a.mass as f64;

            let m_r: Vec3F64 = (a.posit * a.mass).into();
            m_r_sum += m_r;
        }

        for w in &self.water {
            mass_sum += MASS_WATER_MOL as f64;

            let mr_o: Vec3F64 = (w.o.posit * O_MASS).into();
            let mr_h0: Vec3F64 = (w.h0.posit * H_MASS).into();
            let mr_h1: Vec3F64 = (w.h1.posit * H_MASS).into();

            m_r_sum += mr_o + mr_h0 + mr_h1;
        }

        if mass_sum <= EPS {
            return;
        }
        let rot_com: Vec3 = (m_r_sum / mass_sum).into();

        // Build inertia tensor I and angular momentum L about r_cm
        let mut i_xx = 0.0;
        let mut i_xy = 0.0;
        let mut i_xz = 0.0;
        let mut i_yy = 0.0;
        let mut i_yz = 0.0;
        let mut i_zz = 0.0;

        let mut L = Vec3::new_zero();

        for a in &self.atoms {
            let m = a.mass;

            let r = a.posit - rot_com;
            let vxr = r.cross(a.vel);
            L += vxr * m;

            let rx = r.x;
            let ry = r.y;
            let rz = r.z;
            let r2 = rx * rx + ry * ry + rz * rz;

            i_xx += m * (r2 - rx * rx);
            i_yy += m * (r2 - ry * ry);
            i_zz += m * (r2 - rz * rz);
            i_xy -= m * (rx * ry);
            i_xz -= m * (rx * rz);
            i_yz -= m * (ry * rz);
        }

        for w in &self.water {
            for a in [&w.o, &w.h0, &w.h1] {
                let m = a.mass;

                let r = a.posit - rot_com;
                let vxr = r.cross(a.vel);
                L += vxr * m;

                let rx = r.x;
                let ry = r.y;
                let rz = r.z;
                let r2 = rx * rx + ry * ry + rz * rz;

                i_xx += m * (r2 - rx * rx);
                i_yy += m * (r2 - ry * ry);
                i_zz += m * (r2 - rz * rz);
                i_xy -= m * (rx * ry);
                i_xz -= m * (rx * rz);
                i_yz -= m * (ry * rz);
            }
        }

        // Inertia tensor (symmetric)
        let I = [[i_xx, i_xy, i_xz], [i_xy, i_yy, i_yz], [i_xz, i_yz, i_zz]];

        // Solve I * ω = L  (3x3). Add tiny Tikhonov for degeneracy.
        let eps = 1.0e-6f32;
        let Ireg = [
            [I[0][0] + eps, I[0][1], I[0][2]],
            [I[1][0], I[1][1] + eps, I[1][2]],
            [I[2][0], I[2][1], I[2][2] + eps],
        ];

        // Inverse of 3x3 (cofactor / det)
        let det = Ireg[0][0] * (Ireg[1][1] * Ireg[2][2] - Ireg[1][2] * Ireg[2][1])
            - Ireg[0][1] * (Ireg[1][0] * Ireg[2][2] - Ireg[1][2] * Ireg[2][0])
            + Ireg[0][2] * (Ireg[1][0] * Ireg[2][1] - Ireg[1][1] * Ireg[2][0]);

        if det.abs() < 1e-12 {
            return;
        }

        let inv_det = 1.0 / det;
        let inv = [
            [
                (Ireg[1][1] * Ireg[2][2] - Ireg[1][2] * Ireg[2][1]) * inv_det,
                (Ireg[0][2] * Ireg[2][1] - Ireg[0][1] * Ireg[2][2]) * inv_det,
                (Ireg[0][1] * Ireg[1][2] - Ireg[0][2] * Ireg[1][1]) * inv_det,
            ],
            [
                (Ireg[1][2] * Ireg[2][0] - Ireg[1][0] * Ireg[2][2]) * inv_det,
                (Ireg[0][0] * Ireg[2][2] - Ireg[0][2] * Ireg[2][0]) * inv_det,
                (Ireg[0][2] * Ireg[1][0] - Ireg[0][0] * Ireg[1][2]) * inv_det,
            ],
            [
                (Ireg[1][0] * Ireg[2][1] - Ireg[1][1] * Ireg[2][0]) * inv_det,
                (Ireg[0][1] * Ireg[2][0] - Ireg[0][0] * Ireg[2][1]) * inv_det,
                (Ireg[0][0] * Ireg[1][1] - Ireg[0][1] * Ireg[1][0]) * inv_det,
            ],
        ];

        let omega = Vec3 {
            x: inv[0][0] * L.x + inv[0][1] * L.y + inv[0][2] * L.z,
            y: inv[1][0] * L.x + inv[1][1] * L.y + inv[1][2] * L.z,
            z: inv[2][0] * L.x + inv[2][1] * L.y + inv[2][2] * L.z,
        };

        // If ω is tiny, nothing to do
        if !omega.x.is_finite() || !omega.y.is_finite() || !omega.z.is_finite() {
            return;
        }
        if omega.magnitude_squared() < 1e-16 {
            return;
        }

        // v' = v - ω × (r - r_cm)
        for a in &mut self.atoms {
            let r = a.posit - rot_com;
            a.vel -= omega.cross(r);
        }

        // todo: Do we need to shake/rattle here? likely.
        for w in &mut self.water {
            let r_o = w.o.posit - rot_com;
            let r_h0 = w.h0.posit - rot_com;
            let r_h1 = w.h1.posit - rot_com;

            w.o.vel -= omega.cross(r_o);
            w.h0.vel -= omega.cross(r_h0);
            w.h1.vel -= omega.cross(r_h1);
        }

        // Clean up any translation introduced by roundoff
        self.zero_linear_momentum();
    }
}