fix: driven atoms now oscillate around their initial equilibrium position

Previously, apply_driving_force set absolute position to A*cos(2π f t + φ),
ignoring the atom's initial coordinates. For atoms not at the origin (e.g.,
atom 120 at x=119), this incorrectly forced them back toward the origin each
step, causing severe distortion and numerical explosion.

Fix: store each driven atom's initial position as eq_pos/eq_x/eq_y/eq_z at
load time; position is now eq + A*cos(2π f t + φ) in all four engines
(Python, C, C++, Fortran).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

engines/cpp/main.cpp

@@ -86,6 +86,7 @@ struct DriverData {
     std::vector<double> phi_x, phi_y, phi_z;  // radians
     std::vector<int> has_period;     // 0=all, 1=limited
     std::vector<double> period_cycles;
+    std::vector<double> eq_x, eq_y, eq_z;           // 平衡位置（初始坐标）
     std::vector<double> freeze_x, freeze_y, freeze_z;
 };
 
@@ -314,6 +315,9 @@ static DriverData read_driver(const std::string &input_dir, const AtomData &atom
             continue;
         }
         d.atom_idx.push_back(idx);
+        d.eq_x.push_back(atoms.pos_0[idx*3+0]);
+        d.eq_y.push_back(atoms.pos_0[idx*3+1]);
+        d.eq_z.push_back(atoms.pos_0[idx*3+2]);
         d.amp_x.push_back(ax); d.amp_y.push_back(ay); d.amp_z.push_back(az);
         d.freq_x.push_back(fx); d.freq_y.push_back(fy); d.freq_z.push_back(fz);
         // Convert degrees to radians
@@ -700,9 +704,9 @@ static void apply_driving_force(
         }
 
         const double TWO_PI = 2.0 * M_PI;
-        double px = drivers.amp_x[d] * std::cos(TWO_PI * drivers.freq_x[d] * t + drivers.phi_x[d]);
-        double py = drivers.amp_y[d] * std::cos(TWO_PI * drivers.freq_y[d] * t + drivers.phi_y[d]);
-        double pz = drivers.amp_z[d] * std::cos(TWO_PI * drivers.freq_z[d] * t + drivers.phi_z[d]);
+        double px = drivers.eq_x[d] + drivers.amp_x[d] * std::cos(TWO_PI * drivers.freq_x[d] * t + drivers.phi_x[d]);
+        double py = drivers.eq_y[d] + drivers.amp_y[d] * std::cos(TWO_PI * drivers.freq_y[d] * t + drivers.phi_y[d]);
+        double pz = drivers.eq_z[d] + drivers.amp_z[d] * std::cos(TWO_PI * drivers.freq_z[d] * t + drivers.phi_z[d]);
         double vpx = -drivers.amp_x[d] * TWO_PI * drivers.freq_x[d] * std::sin(TWO_PI * drivers.freq_x[d] * t + drivers.phi_x[d]);
         double vpy = -drivers.amp_y[d] * TWO_PI * drivers.freq_y[d] * std::sin(TWO_PI * drivers.freq_y[d] * t + drivers.phi_y[d]);
         double vpz = -drivers.amp_z[d] * TWO_PI * drivers.freq_z[d] * std::sin(TWO_PI * drivers.freq_z[d] * t + drivers.phi_z[d]);