ENCUTGWSOFT

ENCUTGWSOFT = [real]

Default: ENCUTGWSOFT	= ENCUTGW[math]\displaystyle{ \times 0.8 }[/math]	for ALGO=ACFDT
	= ENCUTGW[math]\displaystyle{ \times 0.8 }[/math]	as of VASP.6.3
	= ENCUTGW	else

Important: For vasp.6.3 and later releases ENCUTGWSOFT always defaults to ENCUTGW[math]\displaystyle{ \times 0.8 }[/math].

Descprition: The flag ENCUTGWSOFT sets the energy cutoff for response function, such that it allows to truncate the Coulomb kernel slowly between the energy specified by ENCUTGWSOFT and ENCUTGW using a cosine window function.

RPA/ACFDT correlation energies converge very slowly with respect to [math]\displaystyle{ \mathbf{G}_{\rm max } }[/math]. Thus VASP automatically extrapolates to the infinite basis set limit using a linear regression to the equation: ^[1]^[2]^[3]

[math]\displaystyle{ E_{\mathrm{c}}({\mathbf{G}})=E_{\mathrm{c}}(\infty)+\frac{A}{{\mathbf{G}}^3} }[/math].

This usually leads to much smoother energy-volume curves in ACFDT calculations and MP2 calculations. The modified Coulomb kernel is in this case: [math]\displaystyle{ v_{G} = \frac{4 \pi e^2} {G^2} \frac{1}{2} \left( 1 + \cos \left( \pi \, \frac{ \frac{\hbar^{2} G^2 }{2 m_e} - \mathrm{ ENCUTGWSOFT} }{ \mathrm{ENCUTGW} - \mathrm{ENCUTGWSOFT}} \right) \right) \qquad \mbox{for} \quad \frac{\hbar^2 G^2 }{2 m_e} \gt \mathrm{ENCUTGWSOFT} }[/math]

If LSCK is set to .TRUE., the squeezed Coulomb kernel is used instead of the cosine window:^[4]

[math]\displaystyle{ v_{G} = 4 \pi e^2 \frac{ (G_{max}-G_{min})(G_{max}-G) }{ (G_{min}^2 - G(2G_{min}-G_{max}))^2 } \qquad \mbox{for} \quad \mathrm{ENCUTGWSOFT}=\frac{\hbar^2G_{min}^2}{2m_e}\lt \frac{\hbar^2 G^2}{2m_e}\lt \frac{\hbar^2G_{max}^2}{2m_e}=\mathrm{ENCUTGW} }[/math]

This kernel squeezes contributions from large wave vectors [math]\displaystyle{ G\gt G_{max} }[/math] into the window given by ENCUTGWSOFT.

Mind: The infinite basis set limit extrapolation for RPA/ACFDT is described in more detail here.

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