Anti-binding combination from the O lone-pairs (pi*), with modest contributions from the H atoms. | 
Anti-bonding combinations of (s)p-orbitals between pairs of O and C atoms. This is not pi* as the orbitals are not parallel. There is a weak pi-bonding component between C and O. |
| | HOMO | LUMO |
| C1 2s | -0.000000 | 0.110030 |
| C1 2px | -0.000000 | 0.549521 |
| C1 2py | -0.000000 | -0.253684 |
| C1 2pz | 0.126696 | -0.000000 |
| C2 2s | 0.000000 | -0.110030 |
| C2 2px | -0.000000 | 0.549521 |
| C2 2py | 0.000000 | 0.253684 |
| C2 2pz | -0.126696 | -0.000000 |
| O3 2s | 0.000000 | 0.106874 |
| O3 2px | 0.000000 | 0.282893 |
| O3 2py | -0.000000 | -0.150309 |
| O3 2pz | -0.646833 | 0.000000 |
| O4 2s | -0.000000 | -0.106874 |
| O4 2px | 0.000000 | 0.282893 |
| O4 2py | 0.000000 | 0.150309 |
| O4 2pz | 0.646833 | 0.000000 |
| H5 1s | 0.181046 | 0.061324 |
| H6 1s | -0.181046 | -0.061324 |
| H7 1s | -0.181046 | 0.061324 |
| H8 1s | 0.181046 | -0.061324 |
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