CLAG(1)

💖 clag — NAME 💖

okay bestie, say hi to clag — the Lagrangian Program written in C. that's it, that's the name, she's giving lowercase-and-unbothered 💅

---

✨ DESCRIPTION ✨

so like, clag takes your one and two particle density matrices plus your one and two electron integrals and forms the lagrangian matrix out of them 🦋

and what's that lagrangian for, you ask? it's used for MCSCF and CI derivative calculations. that's the whole vibe. she's a behind-the-scenes girlie making the derivatives happen ✨

---

🎀 REFERENCES 🎀

Lagrangian:

1. A New Dimension to Quantum Chemistry, Y. Yamaguchi, Y. Osamura, J.D. Goddard, H. F. Schaefer III, Oxford Univ. Press, (1994).

citation girlies stay winning, no cap 📚

---

🌸 FILES REQUIRED 🌸

like, she literally cannot do her thing without these, bestie:

• input.dat — Input file
• FILE71 — Transformed one-electron integrals
• FILE72 — Transformed two-electron integrals
• FILE73 — One-Particle Density Matrix
• FILE74 — Two-Particle Density Matrix

---

💜 FILES UPDATED 💜

these ones get touched. changed. upgraded. glow-up'd 🫶

• FILE75 — Lagrangian
• output.dat

---

👑 INPUT FORMAT 👑

okay so the input for this program is read from the file input.dat. here are the keywords that are valid — and yes, every single one, we're not skipping anything 💅

• PRINT = integer — this one controls how chatty the output is, basically the verbosity setting ✨
  • a value of 0 prints nothing (silent queen 🎀)
  • a value of 1 prints calculated ci energy and the various components of the ci energy
  • a value of 2 prints the one- and two-electron contributions to the MO lagrangian as well as the MO lagrangian
  • a value of 3 prints the one-particle density matrix
  • a value of 4 prints the two-particle density matrix

🚨 bestie WARNING 🚨 do not use print = 4 unless the test case is very small. like, genuinely. she will simply produce so much output. you have been warned 😭

• WRITE_CAS_FILES = boolean — if TRUE, then write out the following files in canonical form (p>=q, r>=s, pq>=rs) in Pitzer order:
  • one-electron integrals (file81)
  • two-electron integrals (file82)
  • one-particle density matrix (file83)
  • two-particle density matrix (file84)
  • lagrangian (file85)

this is temporary and is intended to aid in linking DETCI with CASSCF. it's giving "we'll fix this later" energy and honestly? respect 🔥

---

okay that's the whole page bestie, you ate that 🍓 this was Psi Release 2.0, PSI COMMANDS FOR UNIX USERS, and the last change was 21 June, 1996 — iconic, historic, literally a legend 💖✨🎀 bye bestie, go form that lagrangian 🦋💅