CaC2O4H2O crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Ca sites. In the first Ca site, Ca(1) is bonded in a 8-coordinate geometry to one O(1), one O(10), one O(3), one O(5), one O(6), one O(7), and two equivalent O(4) atoms. The Ca(1)-O(1) bond length is 2.43 Å. The Ca(1)-O(10) bond length is 2.41 Å. The Ca(1)-O(3) bond length is 2.46 Å. The Ca(1)-O(5) bond length is 2.51 Å. The Ca(1)-O(6) bond length is 2.44 Å. The Ca(1)-O(7) bond length is 2.50 Å. There is one shorter (2.42 Å) and one longer (2.45 Å) Ca(1)-O(4) bond length. In the second Ca site, Ca(2) is bonded in a 8-coordinate geometry to one O(1), one O(3), one O(6), one O(7), one O(8), one O(9), and two equivalent O(2) atoms. The Ca(2)-O(1) bond length is 2.46 Å. The Ca(2)-O(3) bond length is 2.44 Å. The Ca(2)-O(6) bond length is 2.45 Å. The Ca(2)-O(7) bond length is 2.43 Å. The Ca(2)-O(8) bond length is 2.47 Å. The Ca(2)-O(9) bond length is 2.56 Å. There is one shorter (2.41 Å) and one longer (2.42 Å) Ca(2)-O(2) bond length. There are four inequivalent C sites. In the first C site, C(1) is bonded in a distorted bent 120 degrees geometry to one O(1) and one O(3) atom. The C(1)-O(1) bond length is 1.27 Å. The C(1)-O(3) bond length is 1.27 Å. In the second C site, C(2) is bonded in a distorted bent 120 degrees geometry to one O(2) and one O(4) atom. The C(2)-O(2) bond length is 1.27 Å. The C(2)-O(4) bond length is 1.27 Å. In the third C site, C(3) is bonded in a distorted bent 120 degrees geometry to one O(5) and one O(6) atom. The C(3)-O(5) bond length is 1.27 Å. The C(3)-O(6) bond length is 1.26 Å. In the fourth C site, C(4) is bonded in a distorted bent 120 degrees geometry to one O(7) and one O(8) atom. The C(4)-O(7) bond length is 1.26 Å. The C(4)-O(8) bond length is 1.27 Å. There are four inequivalent H sites. In the first H site, H(1) is bonded in a distorted single-bond geometry to one O(8) and one O(9) atom. The H(1)-O(8) bond length is 1.64 Å. The H(1)-O(9) bond length is 1.01 Å. In the second H site, H(2) is bonded in a single-bond geometry to one O(9) atom. The H(2)-O(9) bond length is 0.98 Å. In the third H site, H(3) is bonded in a single-bond geometry to one O(10) atom. The H(3)-O(10) bond length is 0.99 Å. In the fourth H site, H(4) is bonded in a single-bond geometry to one O(10) atom. The H(4)-O(10) bond length is 0.99 Å. There are ten inequivalent O sites. In the first O site, O(1) is bonded in a distorted single-bond geometry to one Ca(1), one Ca(2), and one C(1) atom. In the second O site, O(2) is bonded in a distorted single-bond geometry to two equivalent Ca(2) and one C(2) atom. In the third O site, O(3) is bonded in a distorted single-bond geometry to one Ca(1), one Ca(2), and one C(1) atom. In the fourth O site, O(4) is bonded in a distorted single-bond geometry to two equivalent Ca(1) and one C(2) atom. In the fifth O site, O(5) is bonded in a distorted single-bond geometry to one Ca(1) and one C(3) atom. In the sixth O site, O(6) is bonded in a distorted single-bond geometry to one Ca(1), one Ca(2), and one C(3) atom. In the seventh O site, O(7) is bonded in a 3-coordinate geometry to one Ca(1), one Ca(2), and one C(4) atom. In the eighth O site, O(8) is bonded in a distorted trigonal planar geometry to one Ca(2), one C(4), and one H(1) atom. In the ninth O site, O(9) is bonded in a water-like geometry to one Ca(2), one H(1), and one H(2) atom. In the tenth O site, O(10) is bonded in a distorted water-like geometry to one Ca(1), one H(3), and one H(4) atom. Linkers: 8 [O]C(=O)C([O])=O. Metal clusters: 8 [Ca]. The MOF has largest included sphere 1.56 A, density 2.20 g/cm3, surface area 2439.93 m2/g, accessible volume 0.05 cm3/g