CaO4C5H6 crystallizes in the trigonal R-3 space group. The structure consists of fifty-four 02329_fluka molecules inside a CaC2O4 framework. In the CaC2O4 framework, Ca(1) is bonded to one O(2), two equivalent O(1), two equivalent O(3), and two equivalent O(4) atoms to form distorted edge-sharing CaO7 pentagonal bipyramids. The Ca(1)-O(2) bond length is 2.31 Å. There is one shorter (2.44 Å) and one longer (2.53 Å) Ca(1)-O(1) bond length. There is one shorter (2.38 Å) and one longer (2.58 Å) Ca(1)-O(3) bond length. There is one shorter (2.29 Å) and one longer (2.45 Å) Ca(1)-O(4) bond length. There are two inequivalent C sites. In the first C site, C(5) is bonded in a distorted bent 120 degrees geometry to one O(3) and one O(4) atom. The C(5)-O(3) bond length is 1.27 Å. The C(5)-O(4) bond length is 1.28 Å. In the second 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 1.29 Å. The C(1)-O(2) bond length is 1.26 Å. There are four inequivalent O sites. In the first O site, O(3) is bonded in a distorted single-bond geometry to two equivalent Ca(1) and one C(5) atom. In the second O site, O(4) is bonded in a 3-coordinate geometry to two equivalent Ca(1) and one C(5) atom. In the third O site, O(1) is bonded in a distorted single-bond geometry to two equivalent Ca(1) and one C(1) atom. In the fourth O site, O(2) is bonded in a bent 150 degrees geometry to one Ca(1) and one C(1) atom. Linkers: 6 [O]C(=O)CCCC([O])=O. Metal clusters: 6 [Ca]. The MOF has largest included sphere 3.87 A, density 1.58 g/cm3, surface area 3811.97 m2/g, accessible volume 0.15 cm3/g