Webinar60 - Orbital Optimized Density Functional Theory for Electronic Excited States

QUICKLINKS:
1.38 References

2.03 Act I: Motivation
2.13 Ground state quantum theory - Kohn Sham (KS) DFT
3.14 Ground state quantum theory - Low order coupled cluster (CC) theory (CCSD, etc.)
3.45 Ground state quantum theory - Multireference methods
4.52 Excited State quantum chemistry
5.10 Excited State quantum chemistry - TDDFT
5.50 Issues with TDDFT
7.08 Orbital optimization to the rescue!
8.28 Not exactly breaking news
8.56 Why don't people orbital optimize everywhere?

10.22 Act II: Ansӓtze
10.31 Ansӓtze: Single Determinants
11.37 Singly Excited Singlets
12.16 Ansӓtze: Two unpaired spins
13.41 Ansӓtze: Three or more unpaired spins
15.30 Ansӓtze summary

16.52 Act III: Algorithms
16.58 Ground State Algorithms: Self-Consistent Field
18.42 Ground State Algorithms: Direct Minimization
20.05 Variational Collapse
20.59 Averting Variational Collapse: Self-Consistent Field
22.47 Square Gradient Minimization
24.07 Square Gradient Minimization: Mathematics
24.59 Square Gradient Minimization: Summary

26.31 Act IV: Applications
26.46 Simple Single Excitations
28.11 Double Excitations
29.43 Long Range Charge Transfer
30.50 Thermally Activated Delayed Fluoresence
32.23 Core-Level Spectroscopy
33.19 Performance of ROKS/SGM
34.43 TDDFT: Prediction of complete spectrum
35.19 ROKS: Prediction of complete spectrum
36.03 Recoupling 3 electrons: Case of Allyl Radical
36.46 Recoupling 3 electrons: Case of CO+
37.41 Heavy Elements: Relativistic Effects

39.08 Act V: How to Use
39.22 Available Features
40.37 What do we need for a calculation?
42.40 Example: ΔSCF 2s2 → 2p2 for Be
44.07 Example: C 1s→ 3p for HCHO
47.33 Practical point: $reorder_mo
49.09 Practical point: Hole Localization
50.25 Some more practical points
52.31 Summary
53.12 Conclusions
54.17 Acknowledgements
54.46 References
54.52 Q&A


Abstract
Density Functional Theory-based modeling of electronic excited states is of importance for the investigation of the photophysical/photochemical properties and spectroscopic characterization of large systems. The widely-used linear response Time-Dependent DFT (TDDFT) approach is not effective at modeling many types of excited states, including charge-transfer states, doubly excited states and core-level excitations.

In this webinar, I will discuss the use of state-specific Orbital Optimized (OO) DFT approaches as an alternative to TDDFT for electronic excited states. I will first discuss the motivation and theory behind such approaches, along with some challenges faced by such methods both historically and at present. Subsequently, I will present the Square Gradient Minimization (SGM) algorithm for reliable and efficient excited state orbital optimization, which has been implemented in Q-Chem. In particular, SGM permits use of Restricted Open-shell Kohn-Sham (ROKS) for modeling arbitrary singlet excited states. ROKS/SGM can thus be used to compute core-excitation energies, with the modern SCAN functional yielding ~ 0.3 eV error vs experiment (compared to ~0.1 eV uncertainty in the experimental values themselves). The use of ROKS/SGM in computing Near-Edge X-ray Absorption Fine Structure (NEXAFS) will be discussed. Time permitting, I would also touch upon the recent implementation of the X2C relativistic model in Q-Chem, which (among other things) can be used to accurately compute NEXAFS of elements as heavy as Cr.

About the Presenter:
Diptarka Hait received his S.B. degree in Chemistry and Physics from MIT in 2016, working in the group of Prof. Troy Van Voorhis and was introduced to the world of OO-DFT. He is currently a PhD candidate in Prof. Martin Head-Gordon's group at the UC Berkeley. He is interested in the development of quantum chemistry methods and their application to problems of interest to the experimental community, with specific emphasis on the use of DFT beyond ground state energetics.