1 // ******************************************************************************
2 //
3 // Title: Force Field X.
4 // Description: Force Field X - Software for Molecular Biophysics.
5 // Copyright: Copyright (c) Michael J. Schnieders 2001-2025.
6 //
7 // This file is part of Force Field X.
8 //
9 // Force Field X is free software; you can redistribute it and/or modify it
10 // under the terms of the GNU General Public License version 3 as published by
11 // the Free Software Foundation.
12 //
13 // Force Field X is distributed in the hope that it will be useful, but WITHOUT
14 // ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
15 // FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
16 // details.
17 //
18 // You should have received a copy of the GNU General Public License along with
19 // Force Field X; if not, write to the Free Software Foundation, Inc., 59 Temple
20 // Place, Suite 330, Boston, MA 02111-1307 USA
21 //
22 // Linking this library statically or dynamically with other modules is making a
23 // combined work based on this library. Thus, the terms and conditions of the
24 // GNU General Public License cover the whole combination.
25 //
26 // As a special exception, the copyright holders of this library give you
27 // permission to link this library with independent modules to produce an
28 // executable, regardless of the license terms of these independent modules, and
29 // to copy and distribute the resulting executable under terms of your choice,
30 // provided that you also meet, for each linked independent module, the terms
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32 // module which is not derived from or based on this library. If you modify this
33 // library, you may extend this exception to your version of the library, but
34 // you are not obligated to do so. If you do not wish to do so, delete this
35 // exception statement from your version.
36 //
37 // ******************************************************************************
38 package ffx.numerics.multipole;
39
40 import jdk.incubator.vector.DoubleVector;
41 import jdk.incubator.vector.VectorOperators;
42
43 import static ffx.numerics.multipole.EwaldTensorGlobal.initEwaldSource;
44 import static ffx.numerics.special.Erf.erfc;
45 import static java.lang.Math.PI;
46 import static org.apache.commons.math3.util.FastMath.sqrt;
47
48 /**
49 * The EwaldMultipoleTensorGlobal class computes derivatives of erfc(<b>r</b>)/|<b>r</b>| via
50 * recursion to arbitrary order for Cartesian multipoles in the global frame.
51 *
52 * @author Michael J. Schnieders
53 * @see <a href="http://doi.org/10.1142/9789812830364_0002" target="_blank"> Matt Challacombe, Eric
54 * Schwegler and Jan Almlof, Modern developments in Hartree-Fock theory: Fast methods for
55 * computing the Coulomb matrix. Computational Chemistry: Review of Current Trends. pp. 53-107,
56 * Ed. J. Leczszynski, World Scientifc, 1996. </a>
57 * @since 1.0
58 */
59 public class EwaldTensorGlobalSIMD extends CoulombTensorGlobalSIMD {
60
61 /**
62 * Constant <code>sqrtPI = sqrt(PI)</code>
63 */
64 private static final double sqrtPI = sqrt(PI);
65
66 /**
67 * These are the "source" terms for the recursion for the screened Coulomb operator erfc(R)/R.
68 */
69 private final double[] ewaldSource;
70
71 /**
72 * A work array for generation of source terms that cannot be vectorized (exp and erfc).
73 */
74 private final double[] work;
75
76 /**
77 * The Ewald convergence parameter.
78 */
79 private final double beta;
80
81 /**
82 * Constructor for EwaldMultipoleTensorGlobal.
83 *
84 * @param order Tensor order.
85 * @param beta The Ewald convergence parameter.
86 */
87 public EwaldTensorGlobalSIMD(int order, double beta) {
88 super(order);
89 this.beta = beta;
90 operator = Operator.SCREENED_COULOMB;
91
92 // Auxiliary terms for screened Coulomb (Sagui et al. Eq. 2.28)
93 ewaldSource = new double[o1];
94 work = new double[o1];
95 initEwaldSource(order, beta, ewaldSource);
96 }
97
98 /**
99 * Generate source terms for the Ewald Challacombe et al. recursion.
100 *
101 * @param T000 Location to store the source terms.
102 */
103 @Override
104 protected void source(DoubleVector[] T000) {
105 // Generate source terms for real space Ewald summation.
106 if (beta > 0.0) {
107 fillEwaldSource(order, beta, ewaldSource, R, T000, work);
108 } else {
109 // For beta = 0, generate tensors for the Coulomb operator.
110 super.source(T000);
111 }
112 }
113
114 /**
115 * Fill the Ewald source terms.
116 *
117 * @param order The order plus one.
118 * @param beta The Ewald convergence parameter.
119 * @param ewaldSource The source terms.
120 * @param R The separation distance.
121 * @param T000 The location to store the source terms.
122 * @param work A work array for generation of source terms that cannot be vectorized.
123 */
124 protected static void fillEwaldSource(int order, double beta, double[] ewaldSource,
125 DoubleVector R, DoubleVector[] T000, double[] work) {
126 // Sagui et al. Eq. 2.22
127 DoubleVector betaR = R.mul(beta);
128 DoubleVector betaR2 = betaR.mul(betaR);
129 DoubleVector iBetaR2 = DoubleVector.broadcast(R.species(), 1.0);
130 iBetaR2 = iBetaR2.div(betaR2.mul(2.0));
131 DoubleVector expBR2 = betaR2.neg().lanewise(VectorOperators.EXP);
132 // Fnc(x^2) = Sqrt(PI) * erfc(x) / (2*x)
133 // where x = Beta*R
134 // Serial portion to handle the erfc.
135 betaR.intoArray(work, 0);
136 for (int i = 0; i < R.length(); i++) {
137 work[i] = erfc(work[i]);
138 }
139 DoubleVector Fnc = DoubleVector.fromArray(R.species(), work, 0);
140
141 Fnc = Fnc.mul(sqrtPI).div(betaR.mul(2.0));
142 for (int n = 0; n <= order; n++) {
143 T000[n] = Fnc.mul(ewaldSource[n]);
144 // Generate F(n+1)c from Fnc (Eq. 2.24 in Sagui et al.)
145 // F(n+1)c = [(2*n+1) Fnc(x) + exp(-x)] / 2x
146 // where x = (Beta*R)^2
147 Fnc = ((Fnc.mul(2.0 * n + 1.0)).add(expBR2)).mul(iBetaR2);
148 }
149 }
150
151 }