Unity3D Move

using UnityEngine;
using System.Collections;

namespace PlayerController
{
    [ RequireComponent( typeof( Rigidbody ) ) ]

    public class PlayerMove : MonoBehaviour
    {
        public      float walkAcceleration = 20f;
        public      float walkDeacceleration = .1f;
        public      float maxWalkSpeed = 5f;
        public      float jumpForce = 150f;
        public      float maxSlope = 75f;

        private     Vector2 horizontalMovement;
        private     bool isMoving = false;
        private     float walkDeaccelerationX = 0f;
        private     float walkDeaccelerationZ = 0f;
        private     float ref_walkDeaccelerationX = 0f;
        private     float ref_walkDeaccelerationZ = 0f;
        private     bool isGrounded = false;

        void Move()
        {
            // Move.
            Vector3 forceDirection = new Vector3(
                Input.GetAxis( "Horizontal" ),
                0f,
                Input.GetAxis( "Vertical" )
                ) * walkAcceleration;

            if( forceDirection != Vector3.zero )
            {
                if( isGrounded ) {
                    rigidbody.AddRelativeForce( forceDirection );
                }
                else {
                    rigidbody.AddRelativeForce( forceDirection * .1f );
                }

                isMoving = true;
            }
            else
            {
                if( isMoving )
                {
                    if( isGrounded )
                    {
                        walkDeaccelerationX = Mathf.SmoothDamp(
                            rigidbody.velocity.x,
                            0f,
                            ref ref_walkDeaccelerationX,
                            walkDeacceleration );

                        walkDeaccelerationZ = Mathf.SmoothDamp(
                            rigidbody.velocity.z,
                            0f,
                            ref ref_walkDeaccelerationZ,
                            walkDeacceleration );

                        rigidbody.velocity = new Vector3(
                            walkDeaccelerationX,
                            rigidbody.velocity.y,
                            walkDeaccelerationZ );

                        if( rigidbody.velocity.x == 0 && rigidbody.velocity.z == 0 ) {
                            isMoving = false;
                        }
                    }
                }
            }

            // Limit the velocity.
            horizontalMovement = new Vector2( rigidbody.velocity.x, rigidbody.velocity.z );

            if( horizontalMovement.sqrMagnitude > maxWalkSpeed * maxWalkSpeed )
            {
                horizontalMovement = horizontalMovement.normalized * maxWalkSpeed;

                rigidbody.velocity = new Vector3(
                    horizontalMovement.x,
                    rigidbody.velocity.y,
                    horizontalMovement.y );
            }
        }

        void Jump()
        {
            if( Input.GetKeyDown( KeyCode.Space ) && isGrounded )
            {
                rigidbody.velocity = new Vector3(
                    rigidbody.velocity.x,
                    0,
                    rigidbody.velocity.z );
            }
        }

        void Start() {
            rigidbody.freezeRotation = true;
        }

        void FixedUpdate()
        {
            Move();
            Jump();
        }

        void OnCollisionStay( Collision collisionInfo )
        {
            foreach( ContactPoint contactPoint in collisionInfo.contacts )
            {
                if( Vector3.Angle( contactPoint.normal, Vector3.up ) <= maxSlope )
                {
                    isGrounded = true;
                }
            }
        }

        void OnCollisionExit()
        {
            isGrounded = false;
        }
    }
}