G2, G2(MP2), and G2(B3LYP/MP2/CC) calculations show the vinylcyclopropenyl cation 3 to be the lowest energy of the C5H5+ isomers. The calculated energy difference between the cyclopentadienyl cation 2 and the vinylcyclopropenyl cation 3 is very small (11.9 and 13.1 kJ mol(-1) at G2 and G2(B3LYP/MP2/CC) levels, respectively), and higher level corrections used in G2, G2(MP2), and G2(B3LYP/MP2/CC) theories have a crucial effect upon it. Calculations using G2 theory without higher level corrections indicate that the cyclopentadienyl cation 2 is 0.2 kJ mol(-1) mow stable than the vinylcyclopropenyl cation 3. The calculated Delta H-f298 values for 2 and 3 are 1090.6 and 1081.1 kJ mol(-1), respectively. The disagreement of the calculated G2 Delta H-f298(2) and Delta H-f298(3) values with the experimental estimates of 1052 and 1012 kJ mol(-1) leads to the suggestion that these experimental estimates need to be. reexamined. The calculated electron affinity for the cation 2 (8.41 eV using G2 theory) is in excellent agreement with the experimental value of 8.41 eV, Calculations of the Delta H-f298 value for the nonclassical pyramidal structure (4) of (CH)(5)(+) lead to the value of 1145.3 kJ mol(-1).