lc = 0.25;
x_center = 0;
y_center = 0;
z_center = 0;

// Define the inner sphere
R = 1;

Point(1) = {x_center, y_center, z_center, lc};
Point(2) = {x_center - R, y_center, z_center, lc};
Point(4) = {x_center, y_center - R, z_center, lc};
Point(5) = {x_center + R, y_center, z_center, lc};
Point(8) = {x_center, y_center, z_center - R, lc};
Point(11) = {x_center, y_center + R, z_center, lc};
Point(14) = {x_center, y_center, z_center + R, lc};
Circle (1) = {2, 1, 4} Plane{0, 0, 1};
Circle (2) = {4, 1, 5} Plane{0, 0, 1};
Circle (3) = {2, 1, 8} Plane{0, 0, 1};
Circle (4) = {4, 1, 8} Plane{0, 0, 1};
Circle (6) = {2, 1, 11} Plane{0, 0, 1};
Circle (7) = {8, 1, 11} Plane{0, 0, 1};
Circle (9) = {2, 1, 14} Plane{0, 0, 1};
Circle (10) = {11, 1, 14} Plane{0, 0, 1};
Circle (13) = {14, 1, 4} Plane{0, 0, 1};
Circle (15) = {8, 1, 5} Plane{0, 0, 1};
Circle (18) = {11, 1, 5} Plane{0, 0, 1};
Circle (21) = {14, 1, 5} Plane{0, 0, 1};
Line Loop (1000005) = {1, 4, -3};
Ruled Surface (5) = {1000005};
Line Loop (1000008) = {3, 7, -6};
Ruled Surface (8) = {1000008};
Line Loop (1000011) = {6, 10, -9};
Ruled Surface (11) = {1000011};
Line Loop (1000014) = {9, 13, -1};
Ruled Surface (14) = {1000014};
Line Loop (1000017) = {-15, -4, 2};
Ruled Surface (17) = {1000017};
Line Loop (1000020) = {-18, -7, 15};
Ruled Surface (20) = {1000020};
Line Loop (1000023) = {-21, -10, 18};
Ruled Surface (23) = {1000023};
Line Loop (1000026) = {-2, -13, 21};
Ruled Surface (26) = {1000026};

Surface Loop(1) = { 5, 8, 11, 14, 17, 20, 23, 26 };

// Define the outer sphere
lc = 2.5;
R = 5;

Point(1001) = {x_center, y_center, z_center, lc};
Point(1002) = {x_center - R, y_center, z_center, lc};
Point(1004) = {x_center, y_center - R, z_center, lc};
Point(1005) = {x_center + R, y_center, z_center, lc};
Point(1008) = {x_center, y_center, z_center - R, lc};
Point(1011) = {x_center, y_center + R, z_center, lc};
Point(1014) = {x_center, y_center, z_center + R, lc};
Circle (1001) = {1002, 1001, 1004} Plane{0, 0, 1};
Circle (1002) = {1004, 1001, 1005} Plane{0, 0, 1};
Circle (1003) = {1002, 1001, 1008} Plane{0, 0, 1};
Circle (1004) = {1004, 1001, 1008} Plane{0, 0, 1};
Circle (1006) = {1002, 1001, 1011} Plane{0, 0, 1};
Circle (1007) = {1008, 1001, 1011} Plane{0, 0, 1};
Circle (1009) = {1002, 1001, 1014} Plane{0, 0, 1};
Circle (1010) = {1011, 1001, 1014} Plane{0, 0, 1};
Circle (1013) = {1014, 1001, 1004} Plane{0, 0, 1};
Circle (1015) = {1008, 1001, 1005} Plane{0, 0, 1};
Circle (1018) = {1011, 1001, 1005} Plane{0, 0, 1};
Circle (1021) = {1014, 1001, 1005} Plane{0, 0, 1};
Line Loop (2000005) = {1001, 1004, -1003};
Ruled Surface (1005) = {2000005};
Line Loop (2000008) = {1003, 1007, -1006};
Ruled Surface (1008) = {2000008};
Line Loop (2000011) = {1006, 1010, -1009};
Ruled Surface (1011) = {2000011};
Line Loop (2000014) = {1009, 1013, -1001};
Ruled Surface (1014) = {2000014};
Line Loop (2000017) = {-1015, -1004, 1002};
Ruled Surface (1017) = {2000017};
Line Loop (2000020) = {-1018, -1007, 1015};
Ruled Surface (1020) = {2000020};
Line Loop (2000023) = {-1021, -1010, 1018};
Ruled Surface (1023) = {2000023};
Line Loop (2000026) = {-1002, -1013, 1021};
Ruled Surface (1026) = {2000026};

Surface Loop(2) = { 1005, 1008, 1011, 1014, 1017, 1020, 1023, 1026 };

// Define the volume between the two spheres
Volume(1) = { 2, 1 };


