diff --git a/ciderpress/lib/mod_cider/CMakeLists.txt b/ciderpress/lib/mod_cider/CMakeLists.txt
index a44d0bc..e6d0ed7 100644
--- a/ciderpress/lib/mod_cider/CMakeLists.txt
+++ b/ciderpress/lib/mod_cider/CMakeLists.txt
@@ -1,6 +1,6 @@
 
 add_library(mcider SHARED
-  frac_lapl.c cider_coefs.c cider_grids.c ../opt_cider/spline.c sph_harm.c
+  frac_lapl.c cider_coefs.c cider_grids.c spline.c sph_harm.c
   conv_interpolation.c convolutions.c fast_sdmx.c pbc_tools.c debug_numint.c
 )
 
@@ -9,5 +9,4 @@ set_target_properties(mcider PROPERTIES
   COMPILE_FLAGS ${OpenMP_C_FLAGS}
   LINK_FLAGS ${OpenMP_C_FLAGS})
 
-target_include_directories(mcider PRIVATE ../opt_cider/)
 target_link_libraries(mcider ${BLAS_LIBRARIES})
diff --git a/ciderpress/lib/mod_cider/spline.c b/ciderpress/lib/mod_cider/spline.c
new file mode 100644
index 0000000..120e47e
--- /dev/null
+++ b/ciderpress/lib/mod_cider/spline.c
@@ -0,0 +1,76 @@
+#include "spline.h"
+#include <math.h>
+#include <stdlib.h>
+
+void get_cubic_spline_coeff(double *x, double *y, double *spline, int N) {
+    double **coeff = (double **)malloc(3 * sizeof(double *));
+    coeff[0] = spline + 2 * N;
+    coeff[1] = spline + 3 * N;
+    coeff[2] = spline + 4 * N;
+
+    int i;
+    for (i = 0; i < N; i++) {
+        spline[i] = x[i];
+        spline[N + i] = y[i];
+    }
+
+    double d1p1 = (y[1] - y[0]) / (x[1] - x[0]);
+    if (d1p1 > 0.99E30) {
+        coeff[1][0] = 0;
+        coeff[0][0] = 0;
+    } else {
+        coeff[1][0] = -0.5;
+        coeff[0][0] =
+            (3 / (x[1] - x[0])) * ((y[1] - y[0]) / (x[1] - x[0]) - d1p1);
+    }
+
+    double s = 0, r = 0;
+
+    for (i = 1; i < N - 1; i++) {
+        s = (x[i] - x[i - 1]) / (x[i + 1] - x[i - 1]);
+        r = s * coeff[1][i - 1] + 2;
+        coeff[1][i] = (s - 1) / r;
+        coeff[0][i] = (6 *
+                           ((y[i + 1] - y[i]) / (x[i + 1] - x[i]) -
+                            (y[i] - y[i - 1]) / (x[i] - x[i - 1])) /
+                           (x[i + 1] - x[i - 1]) -
+                       s * coeff[0][i - 1]) /
+                      r;
+    }
+
+    coeff[0][N - 1] = 0;
+    coeff[1][N - 1] = 0;
+    coeff[2][N - 1] = 0;
+
+    for (i = N - 2; i >= 0; i--) {
+        coeff[1][i] = coeff[1][i] * coeff[1][i + 1] + coeff[0][i];
+    }
+
+    for (i = 0; i < N - 1; i++) {
+        s = x[i + 1] - x[i];
+        r = (coeff[1][i + 1] - coeff[1][i]) / 6;
+        coeff[2][i] = r / s;
+        coeff[1][i] = coeff[1][i] / 2;
+        coeff[0][i] = (y[i + 1] - y[i]) / s - (coeff[1][i] + r) * s;
+    }
+
+    free(coeff);
+}
+
+double spline_integral(double *spline, int N) {
+    double *x = spline;
+    double *a = spline + N;
+    double *b = spline + 2 * N;
+    double *c = spline + 3 * N;
+    double *d = spline + 4 * N;
+    double dx = 0;
+    double integral = 0;
+    int i;
+    for (i = 0; i < N - 1; i++) {
+        dx = x[i + 1] - x[i];
+        integral +=
+            dx * (a[i] + dx * (b[i] / 2 + dx * (c[i] / 3 + d[i] * dx / 4)));
+    }
+
+    return integral;
+}
diff --git a/ciderpress/lib/mod_cider/spline.h b/ciderpress/lib/mod_cider/spline.h
new file mode 100644
index 0000000..cab7e53
--- /dev/null
+++ b/ciderpress/lib/mod_cider/spline.h
@@ -0,0 +1,7 @@
+#ifndef CIDER_SPLINE_H
+#define CIDER_SPLINE_H
+
+void get_cubic_spline_coeff(double *x, double *y, double *spline, int N);
+double spline_integral(double *spline, int N);
+
+#endif