CaC4H4O6 crystallizes in the monoclinic C2/c space group. Ca(1) is bonded in a 8-coordinate geometry to one O(2), one O(3), one O(4), one O(5), two equivalent O(1), and two equivalent O(6) atoms. The Ca(1)-O(2) bond length is 2.70 Å. The Ca(1)-O(3) bond length is 2.46 Å. The Ca(1)-O(4) bond length is 2.48 Å. The Ca(1)-O(5) bond length is 2.40 Å. There is one shorter (2.43 Å) and one longer (2.44 Å) Ca(1)-O(1) bond length. There is one shorter (2.41 Å) and one longer (2.52 Å) Ca(1)-O(6) 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(2) atom. The C(1)-O(1) bond length is 1.28 Å. The C(1)-O(2) bond length is 1.26 Å. In the second C site, C(2) is bonded in a distorted water-like geometry to one H(3) and one O(3) atom. The C(2)-H(3) bond length is 1.10 Å. The C(2)-O(3) bond length is 1.42 Å. In the third C site, C(3) is bonded in a distorted water-like geometry to one H(4) and one O(4) atom. The C(3)-H(4) bond length is 1.10 Å. The C(3)-O(4) bond length is 1.42 Å. In the fourth C site, C(4) is bonded in a distorted bent 120 degrees geometry to one O(5) and one O(6) atom. The C(4)-O(5) bond length is 1.27 Å. The C(4)-O(6) bond length is 1.28 Å. There are four inequivalent H sites. In the first H site, H(1) is bonded in a distorted linear geometry to one O(2) and one O(3) atom. The H(1)-O(2) bond length is 1.60 Å. The H(1)-O(3) bond length is 1.01 Å. In the second H site, H(2) is bonded in a distorted single-bond geometry to one O(4) and one O(5) atom. The H(2)-O(4) bond length is 1.00 Å. The H(2)-O(5) bond length is 1.69 Å. In the third H site, H(3) is bonded in a single-bond geometry to one C(2) atom. In the fourth H site, H(4) is bonded in a single-bond geometry to one C(3) atom. There are six inequivalent O sites. In the first O site, O(1) is bonded in a distorted single-bond geometry to two equivalent Ca(1) and one C(1) atom. In the second O site, O(2) is bonded in a 2-coordinate geometry to one Ca(1), one C(1), and one H(1) atom. In the third O site, O(3) is bonded in a 2-coordinate geometry to one Ca(1), one C(2), and one H(1) atom. In the fourth O site, O(4) is bonded in a 2-coordinate geometry to one Ca(1), one C(3), and one H(2) atom. In the fifth O site, O(5) is bonded in a distorted single-bond geometry to one Ca(1), one C(4), and one H(2) atom. In the sixth O site, O(6) is bonded in a distorted single-bond geometry to two equivalent Ca(1) and one C(4) atom. Linkers: 4 [O]C(=O)[C@@H](O)[C@@H](O)C([O])=O. Metal clusters: 4 [Ca]. The MOF has largest included sphere 1.62 A, density 2.22 g/cm3, surface area 2105.34 m2/g, accessible volume 0.04 cm3/g