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exercises:common:sgcp

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In this exercise, you will perform ab initio molecular dynamics using Second Generation Car-Parrinello (SGCP) molecular dynamics.

Please cite Phys. Rev. Lett. 98, 066401 , if you use this method.

Published work using SGCP method:

J. Phys. Chem. C 2018, 122, 42, 24068–24076

J. Phys. Chem. Lett. 2020, 11, 9, 3724–3730

1. Introduction

Second Generation CPMD (2ndG CPMD) is a molecular dynamics method that combines the efficiency of Car-Parrinello MD (CPMD) with the accuracy of Born-Oppenheimer MD (BOMD). It avoids fully self-consistent field (SCF) optimizations at each time step while enabling larger integration steps and maintaining accuracy close to BOMD.

Goal: Retain the efficiency of CPMD while achieving BOMD-level accuracy.

- Efficiency: Large time steps ; No full SCF loops

- Accuracy: Forces nearly indistinguishable from BOMD

- Stability: Effective for systems with vanishing band gaps

- Error Control: Controlled deviation from BO surface using adaptive correction

2. Comparison with CPMD and BOMD

Feature CPMD BOMD SGCP
SCF at each step No Yes Partially (predictor-corrector)
Time step Small (~0.1 fs) Large (~1 fs) Large (~1–2 fs)
Conserved quantity preservation Excellent Reasonable Excellent
On Born-Oppenheimer surface Slightly above Yes Very close
Works for small-gap systems Poor Good Good

3. ASPC Method

ASPC Method: Always Stable Predictor Corrector

ASPC is a Gear-type integrator for electronic wavefunctions:

Predictor:

Cp(tn)=Km=1(1)m+1mBmPS(tnm)

where: - Bm: Kolafa predictor coefficients - PS: projection onto the overlap matrix S

Corrector:

C(tn)=ωmin[Cp(tn)]+(1ω)Cp(tn),ω=K2K1

Langevin Dynamics & Dissipation Compensation

Because ASPC introduces small dissipation, Langevin-type equations are used to stabilize the dynamics:

MI¨RI=FBO(γD+γL)˙RI+ΞI

- γD: implicit friction from ASPC - γL: Langevin thermostat - ΞI: Langevin random noise

4. How to Set Up in CP2K

### 1. ASPC Extrapolation

```fortran &FORCE_EVAL

&DFT
  &QS
    EXTRAPOLATION ASPC
    EXTRAPOLATION_ORDER 1
  &END QS
  &SCF
    MAX_SCF_HIST 2
  &END SCF
&END DFT

&END FORCE_EVAL ```

### 2. Langevin Thermostat

```fortran &MOTION

&MD
  ENSEMBLE LANGEVIN
  &LANGEVIN
    GAMMA 0.005         ! γ_L
    NOISY_GAMMA 4.0E-4  ! γ_D
  &END LANGEVIN
&END MD

&END MOTION ```

### 3. Atom-Specific γ_D (Optional)

```fortran &THERMAL_REGION

DO_LANGEVIN_DEFAULT TRUE
&DEFINE_REGION
  TEMPERATURE 500
  NOISY_GAMMA_REGION 4.E-4
  LIST 577..745
&END DEFINE_REGION

&END THERMAL_REGION ```

exercises/common/sgcp.1750300439.txt.gz · Last modified: by jglan