More displacements to obtain precise frequencies

You can specify displacements more than independent freedom to obtain precise eigenvalues (frequencies). This treatment works especially near $\Gamma$-point. An effective example is $\pm$ displacements. If your DISP file is,

	  1   1.00   0.00   0.00
	  1   0.00   1.00   0.00
	  1   0.00   0.00   1.00

You can create displacements by hand (or use sed), e.g.,

	  1   1.00   0.00   0.00
	  1  -1.00   0.00   0.00
	  1   0.00   1.00   0.00
	  1   0.00  -1.00   0.00
	  1   0.00   0.00   1.00
	  1   0.00   0.00  -1.00

This setting may be called 'plus-minus'. The plus-minus setting works to increase stability of phonon band structure even when the initial atomic geometry is not enough converged to the equilibrium position. Especially frequency equivalent to approximately zero is sensitive to this treatment. Sometimes fails when forces are converged too much.