Density functional calculations on carbonic acid H2CO3 are extended to clusters of up to five such units. The most stable forms are the linear, hydrogen-bonded analogs of the dimer with anti-anti orientation. We calculate structures and vibration frequencies, as well as the energy required to bend and stretch the linear isomers. Linear chains of up to similar to 20 units should be favored over ring structures, and they have a tensile strength reminiscent of chains of water molecules. We also discuss planar, nonlinear structures as well as three-dimensional isomers. (C) 2000 American Institute of Physics. [S0021-9606(00)30615-8].