The order of the nodes differs from that accepted in the manual
Create tetrahedron.geo
// single element tetrahedron Point(0) = {0,0,0,10}; Point(1) = {1,0,0,10}; Point(2) = {0,1,0,10}; Point(3) = {0,0,1,10}; Line(4) = {0,1}; Line(5) = {0,2}; Line(6) = {0,3}; Line(7) = {1,2}; Line(8) = {1,3}; Line(9) = {2,3}; Line Loop(10) = {4,7,-5}; // 0-1-2 Line Loop(11) = {4,8,-6}; // 0-1-3 Line Loop(12) = {5,9,-6}; // 0-2-3 Line Loop(13) = {7,9,-8}; // 1-2-3 Plane Surface(14) = {10}; Plane Surface(15) = {11}; Plane Surface(16) = {12}; Plane Surface(17) = {13}; Surface Loop(18) = {14,15,16,17}; Volume(19) = {18};
Generate tetrahedron.msh and compare with the manual