stable(1)

💖 NAME 💖

ok so this is stable — the MO Hessian stability analysis program, section 1 of the manual, she's an icon, she's a classic 💅

✨ DESCRIPTION ✨

okay bestie, listen. stable does a stability analysis on your molecular orbitals by diagonalizing the molecular orbital Hessian. that's the whole vibe. and the analysis includes orbital rotations that may break spin symmetry or spatial symmetry — like, it's not shy, it checks those too 🦋

so if an instability gets detected (translation: there are negative eigenvalues of the MO Hessian), that means your previous Hartree-Fock self consistent field (SCF) computation landed on a local minimum, NOT the global minimum, in orbital rotation space. like, she found a valley, just not the valley. 😭

here's what each flavor of instability is actually telling you:

• for spatial- or symmetry-breaking rotations, it merely means there's another orbital occupation, or a UHF</u> wavefunction, sitting there with a lower energy. no drama, just options.
• for instabilities that preserve the reference type (RHF or UHF) and are totally symmetric, this suggests the SCF computation has simply found the wrong solution and should be re-run using a new initial guess. yeah. it fumbled. re-roll. 🎲

🛠️ how to actually fix it

Instabilities corresponding to spatial symmetry breaking rotations can be resolved by re-running the calculation in a lower computational point group. (and yeah, it may also be necessary to employ some trick to break the symmetry of the initial guess wavefunction. iykyk 🫶)

Instabilities corresponding to spin symmetry breaking rotations (<code>RHF</code> -> <code>UHF</code>) can be resolved by re-running the SCF using a UHF reference. slay, just switch it up ✨

and for UHF wavefunctions with totally-symmetric instabilities, you can actually follow the instability automatically toward the global minimum by setting FOLLOW=TRUE. yes bestie, autopilot. 💅

🎀 INPUT FORMAT 🎀

Input for this program is read from the file 🌸 The following keywords are valid:

• CACHELEV = integer — this is the cache level used for the DPD files. it functions the same way as in the coupled-cluster codes, no cap. currently, this is hardwired to 0. (so like, she takes the input but doesn't take the input, yk? 😭)
• FOLLOW = boolean — if TRUE, then the eigenvector corresponding to the most negative eigenvalue of the MO Hessian will be followed to try to find a lower energy Hartree-Fock solution 🔥 The orbitals will be rotated, and the cscf module should be re-run using these new guess orbitals. main character energy fr fr 👑
• NUM_EVECS_PRINT = integer — gives the number of MO Hessian eigenvectors to print. that's it. that's the job. 💅
• PRINT = integer — the print level determines how much information is printed by the program. values typically range from 1 (minimal printing) to 5 (very verbose printing for debugging only). the default is 1. 🌈
• REFERENCE = string — this is the reference type: RHF, ROHF, or UHF. the default is RHF. ✨
• SCALE = real — this is the scale factor used for taking steps in eigenvector following. normal values would be between 0 and 1. the default is 0.5. 🎀

and that's the whole page, bestie 🫶 this was the stable(1) manual, dated 23 Aug, 2003 — yeah she's vintage, she's retro, she still ate 💖 bye mwah 💅🌸