The predicted crystal structure of 1:1 pyridine…formic acid has a P21/n space-group with Z=4, a=10.88 Å, b=3.91 Å, c= 15.73 Å and α=90.0 o , β=106.9 o and γ=90 o . This is in excellent agreement with the reported experimental crystal-structure (see Supp. Info. Table ). Calculations revealed another orthorhombic crystal with P21212 space-group (Z=4) that is 2.9 kcal/mol higher in energy than the reported structure (Supp. Info. File, Figure ). In all the four cases, the acid and base formed hydrogen-bonded complexes and therefore, are formally termed as co-crystals. The available experimental crystal structures for 1:1 pyridine…formic acid and the 1:2 pyrazine…formic acid adducts at ambient conditions are in agreement with this classification. Cambridge structural database (CSD) did not reveal any crystals for the 1:1 complexes for pyrazine…formic acid, pyridazine…formic acid and furan…formic acid. Recent pulsed-jet Fourier transform microwave spectroscopy on the furan…formic-acid complex indicated a strong O-H…O and weak C-H…O hydrogenbonding. The important hydrogen-bonding patterns for the adducts within the 0 GPa structures are shown in Figure . For comparison, the gas-phase molecular 1:1 analogues are also computed at the B3LYP-D3/def2-TZVPP level in Gaussian program package 66 (See Supp.