This equation describes how DM reaches dynamical equilibrium in the collapsing system in the expanding universe. If the DE is distributed homogeneously, σ$_{d}$ = 0, this equation reduces to For a system in equilibrium, ˙ T$_{m}$ = ˙ U$_{mm}$ = 0, we get the virial condition. If we take ¯ ξ$_{1}$ = 3 ξ$_{1}$, ¯ ξ$_{2}$ = 3 ξ$_{2}$ , we have the virial condition (2 + ¯ ξ$_{1}$ + ¯ ξ$_{2}$/r ) T$_{m}$ + (1 − 2 ¯ ξ$_{1}$ ) U$_{mm}$ = 0 [23]. Neglecting the interaction ¯ ξ$_{1}$ = ¯ ξ$_{2}$ = 0, we recover the usual virial condition obtained in [38]. One can see that the presence of the coupling between DE and DM changes both the time required by the system to reach equilibrium and the equilibrium configuration itself [23]. Now we consider the case that DE is no longer homogeneous and has perturbation. The rate of change of the peculiar velocity of DE is described by the second equation in (8). Multiplying both sides of this equation by aV$_{d}$ρ$_{d}$ ˆ ε and integrating over the volume, on the LHS we have On the RHS the first term reads, For the remaining terms, we have Combining Eqs.(18,19,20), we arrive at describing how the DE reaches dynamical equilibrium in the collapsing system in an expanding universe if it partici- pates in the structure formation. Looking at equations (16) and (21), we see that in non-interacting case ( ξ$_{1}$ = ξ$_{2}$ = 0), when C 2 e = 0, DE would cluster just like cold DM. Examples of DE models with this property were investigated in [37]. In interacting case, we see that for a collapsing system to reach dynamical equilibrium, the time and dynamics required by DE and DM to 7 ∂ ∂t ( a $^{2}$T$_{d}$ ) + 3 a $^{2}$H ( w + ξ$_{1}$r + ξ$_{2}$ ) T$_{d}$ . (18) − ∫ aV$_{d}$ ∇$_{x}$ ( aψ$_{m}$ + aψ$_{d}$ ) ρ$_{d}$ ˆ ε = − a 2 1 + w ( ˙ U$_{dd}$ + HU$_{dd}$ ) − a 2 1 + w ∫ ψ$_{m}$ ∂ ∂t ( σ$_{d}$ ˆ ε ) − a 2 1 + w 3 H { 2( C 2 e + ξ$_{2}$ ) U$_{dd}$ + 2 ξ$_{1}$U$_{mm}$ + ξ$_{1}$U$_{md}$ + ( C 2 e + ξ$_{2}$ ) U$_{dm}$ } . (19) − c 2 e 1 + w ∫ aV$_{d}$ ∇$_{x}$ ( a Δ$_{d}$) ρ$_{d}$ ˆ ε + 3 H [ ( w − c 2 $_{a}$) + 1 + w − C 2 a 1 + w ( ξ$_{1}$r + ξ$_{2}$ ) ] ∫ ( aV$_{d}$ ) $^{2}$ρ$_{d}$ ˆ ε = − c 2 e 1 + w a 2 ∫ V$_{d}$ ∇$_{x}$ ( σ$_{d}$ )ˆ ε + 6 a $^{2}$H [ ( w − C 2 $_{a}$) + 1 + w − c 2 a 1 + w ( ξ$_{1}$r + ξ$_{2}$ ) ] T$_{d}$ . (20) (1 + w ) ˙ T$_{d}$ + ˙ U$_{dd}$ + H [2(1 + w ) T$_{d}$ + U$_{dd}$ ] = − 3 H { 2( C 2 e + ξ$_{2}$ ) U$_{dd}$ + 2 ξ$_{1}$U$_{mm}$ + ξ$_{1}$U$_{md}$ + ( C 2 e + ξ$_{2}$ ) U$_{dm}$ } − ∫ ψ$_{m}$ ∂ ∂t ( σ$_{d}$ ˆ ε ) − c 2 e ∫ V$_{d}$ ∇$_{x}$ ( σ$_{d}$ )ˆ ε + 3 H [ (1 + w )( w − 2 C 2 $_{a}$) + (1 + w − 2 C 2 $_{a}$)( ξ$_{1}$r + ξ$_{2}$ ) ] T$_{d}$, (21) ˙ T$_{m}$ + ˙ U$_{mm}$ + H (2 T$_{m}$ + U$_{mm}$ ) = − 3 H ( ξ$_{1}$ + ξ$_{2}$/r ) T$_{m}$ + 6 Hξ$_{1}$U$_{mm}$. (17)